Seems like Serbia’s position is that Croatia owns it and Serbia has no intent to administer it given that, while Croatia’s position is that (1) either Serbia or Croatia owns it, (2) it's probably Serbia, and Croatia makes no positive claim, but also won’t force the issue pending international arbitration to clarify the border, and (3) Criatia will continue administering it as an interim matter until the border is resolved.

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.