> For each "disputed" territory there is only one party actually in control right now. If you bothered to actually travel to e.g. Taiwan you'd quickly find out that the PRC's opinions are irrelevant.
If you bothered to actually travel to Ukraine you'd find there are plenty of places where you can get dronestruck by either side. If you bothered to actually travel to the disputed India/China/Pakistan border area you'll find either side might hit you with a stick and take you into custody. If you'd lived in Northern Ireland 40 years ago, or even South Italy today, you'd find that the group that enforces its laws with force in your street might be quite different from the group that is internationally thought to be "in control" of the country you're supposedly part of.
Control is not a binary. There's no country in the world that doesn't have murders, kidnappings, and takeovers. When (as in South Korea recently) there's a group of people outside a guy's house who say they're coming to arrest him under a warrant from the Supreme Court, and he says he's the President and the warrant is illegitimate and they're just a gang of thugs trying to kidnap him, who is in control? There's no way to answer that except retrospectively. When armed men declare independence or a revolution, are they "in control" of an area in a sense that should be shown on a map, or just regular criminals? Again, no way to tell.
I'm quite familiar with Taiwan thank you very much, and yes it's one of the more stable situations (although these things have a way of seeming stable until they suddenly aren't) where one of the claims is rather attenuated and silly. That doesn't mean there's a simple, easy solution to territory disputes in general.