So it seems clear that (1) its either non-ceded Croatian territory being administered by Croatia with a diplomatic choice not to make an affirmative claim without arbitration of the border, or (2) it's by prior treaty Serbian territory, which may or may have been ceded to Croatia, which is being administered by Croatia without challenge to any rights Serbia (and only Serbia) may have.
Furthermore international law differs from normal law as usually at least one party has tanks.
But there's no dispute that either Serbia or Croatia owns it, and it doesn't really matter, as to any third party attempt to set up a state in defiance of the status quo administration, whether Croatia is conducting non-hostile administration of Serbian territory with Serbian consent or administering its own territory.
If there's a smudge in the only copy of a contract you can find and you think it called for a 60/40 split of profits from a certain kind of joint activity in your partner’s favor, and they think it called for a 60/40 split in your favor, there's 20% that is uncertain until some reolution is reached, but that doesn’t mean that someone else can just restructure the deal unilaterally to be 40/40/20, which is (but for the actual numbers and the nonfungible nature of land) what Liberland is trying to do.
The Liberlanders' claim that the patch was terra nullius was silly from day one. An area is terra nullius if there is no recognized government hoping to run it, not if there is no recognized government that will publicly commit to keep running it forever.
The Liberlanders' new claim that the patch is bona vacantia is just completely batshit gaga. Bona vacantia is about private ownership title rather than sovereignty, does not exist in civil law jurisdictions, and when successfully invoked just means ownership of the item in question has reverted to the Crown.
We need a moratorium on ancaps having legal opinions until their semantics games become functionally distinguishable from SovCit woo.
So, neither side claims it, because they both claim a better area. But whoever ends up losing the better area is going to take it.