The whole issue of Sark is rather rather bizarre. Not that I am complaining to have increased my number of countries by one (compensating for Kosovo, which is still not in the list ...), but I am not aware of any change in the status of the island that would have called for making it a top level administrative unit. Why is Sark now a "country" of its own? Compared to Alderney and e.g. Herm I cannot see any reason for this.
Apart from the missing flag on PGC there are more issues with it: not all caches on Sark have been transferred to the new "country". Sark is also still considered on PGC as one of the three "regions" of Guernsey, and so I have 2 finds on Sark counting for this third region of Guernsey. My original guess was that this is the case for all caches on Sark that had been archived before the introduction of the new "country" Sark on geocaching.com, but according to the forum discussion linked by Optimist on the run this (2.8.2018) took place before the archiving of at least one of them (GC5TZ1Y, archived on 22.9.2018). Still, all those caches still being counted as belonging to Guernsey, are archived ones.
BTW, this confusion is another argument for a suggestion I have made before, namely not only to overrule the CO’s attribution to a region on PGC but also to a country.