After a lapse of a few days, here, for your revived consideration, is
a presentation of another enclave that went away and came back. you
didn't like the pope's possession as an example, so how about this one?

The Walled City of Kowloon inside British leased Hong Kong New
Territories.

Here's a short history:

June 9, 1898 Convention Respecting an Extension of the Hong Kong
Territory signed in Peking, provided that:

- with respect to the walled city (Kowloon) "...Chinese officials
now stationed there shall continue to exercise jurisdiction except as
may be inconsistent with the military requirements for the defense of
Hong Kong. Within the remainder of the newly-leased territory Great
Britain shall have sole jurisdiction. Chinese officials and people
shall be allowed as heretofore to use the road from Kowloon to Hsinan."

- "It is further agreed that the existing landing-place near
Kowloon City shall be reserved for the convenience of Chinese
men-of-war, merchant and passenger vessels which may lie there and
come and go at their pleasure; and for the convenience of movement of
the officials and people within the city."

August 6, 1898, Ratifications exchanged in London.

October 20, 1898 New Territories Order in Council (Court at Balmoral)
ordered (in its paragraph 4):

- Notwithstanding anything contained herein, the Chinese
officials now stationed within the City of Kowloon shall continue to
exercize jurisdiction therein except in so far as may be inconsistent
with the military requirements for the defense of Hong Kong.."

December 27, 1899 Walled City Order in Council (Court at Windsor) ordered:

- "...Article 4 of the Order of Her Majesty in Council of the
20th day of October, 1898, is hereby revoked... The City of Kowloon
shall be, and the same is hereby declared, for the term of the
lease... part and parcel of Her Majesty's Colony of Hong Kong..."

"After the war the Chinese government planned to restore her
administration and the provincial authorities announced intention to
establish Chinese civil courts there" [Hong Kong Telegraph Dec. 6,
1947].

During its occupation of Hong Kong (24 Dec 1941 - Aug 1945), Japan
evicted people from the city; during the Japanese occupation the area
was sparsely populated. In 1943 the walls were demolished to provide
material for Kai Tak Airport improvements. After Japan's surrender,
squatters (whether former residents or - more likely - newcomers)
began to occupy the Walled City, resisting several attempts by Britain
in 1948 to drive them out. "The exact boundaries of the Walled City
cannot now be determined". (Wesley-Smith, Unequal Treaty). With no
wall to protect it (initially), the Walled City became a haven for
crooks and drug addicts, as the Hong Kong Police had no right to enter
the City (and mainland China refused to take care of it).

The 1949 foundation of the Peoples' Republic of China added thousands
of refugees to the population, many from Guangzhou, and by this time,
Britain had had enough, and simply adopted a 'hands-off' policy. A
murder that occurred in Kowloon in 1959 set off a small diplomatic
crisis, as the two nations each tried to get the other to claim
responsibility for a vast tract of land now virtually ruled by
anti-Manchurian Triads. (The Triad is a collective term that
describes many branches of the underground society based in Hong
Kong). The Triads' rule lasted up until the mid-1970s, when a series
of over 3,000 police raids occurred in Kowloon. With the Triads' power
diminished, a strange sort of synergy blossomed, and the Walled City
began to grow almost organically, the square buildings folding up into
one another as thousands of modifications were made, virtually none by
architects, until hundreds of square metres were simply a kind of
patchwork monolith. Labyrinthine corridors ran through the monolith,
some of those being former streets (at the ground level, and often
clogged up with trash), and some of those running through upper
floors, practically between buildings. The only rules of construction
were twofold: electricity had to be provided to avoid fire, and the
buildings could be no more than about fourteen stories high (because
of the nearby airport). A mere eight municipal pipes somehow provided
water to the entire structure (although more could have come from
wells). By the early 1980s, Kowloon had an estimated population of
35,000 - with a crime rate far below the Hong Kong average, despite
the notable lack of any real law enforcement.

Over time, both the British and Chinese governments found this
massive, anarchic city to be a bit much - despite the low crime, if
the 'Black Market' ever had a physical location, this would have been
it, and needless to say, the sanitary conditions were, well, a bit
wanting. [Some Post WWII History above from the "Free Dictionary.com".]

April 24, 1975, Hong Kong officials quoted as saying "Walled City is
not under the jurisdiction of the [Hong Kong] government" (South China
Morning Post).

After the Joint Declaration in 1984 The Sino-British Joint Declaration
on the Question of Hong Kong (The Joint Declaration), was signed by
the Prime Ministers of the People's Republic of China (PRC) and the
United Kingdom (UK) governments on December 19, 1984 in Beijing. The
Declaration entered into force with the exchange of instruments of
ratification on May 27, 1985 and was registered by the PRC and UK
governments at the United Nations on June 12, 1985. After the joint
declaration in 1984, China allowed British authorities to demolish the
City and resettle its inhabitants. The mutual decision to tear down
the walled city was made in 1987.

Summary:
From the De Jure standpoint:

a. From the HKK-British paradigm, the enclave existed for about
14 months (a little longer from the British home government that was
not dependent upon the New Territories Orders), 1898-99.

b. From the Chinese standpoint, it started to exist as non-leased
enclave within leased New Territories at start of lease 1898; it
ceased to exist as sovereignly differentiatable from surrounding
occupied territory only during Japanese occupation 1941-5 and it
returned to exist as non-leased territory surrounded by leased
trerritory in 1945 when GB power returned so that exercise of lease
terms could be resumed.

From the de facto standpoint, Kowloon existed as enclave within the
leased area under British sovereignty from beginning of the New
Territories lease until revocation in 1899, was in limbo because the
weak Chinese government of the time could not exercise objections to
British actions until the Japanese took it in 1941 when it ceased to
be either under British or Chinese control. It formed again when the
Japanese left and lease terms resumed, but was under nominal Chinese
"control" until lease ended with British exercising minor
administrative power when defense (civil and military) of the leased
territory required (under the original provision of the lease from 1898).

One can say that sovereignty was shared in certain aspects at certain
times, but as an international enclave that existed and that came and
went and returned (from at lease someone's official sovereign
standpoint) can't be denied.

LN