The constitution of Taiwan claims sovereignty over mainland China, although they added an amendment to differentiate “free area” citizens (Taiwanese) and citizens under control of the “rebellions” (I don’t know if they removed the specific wording yet).
You could argue that Taiwan have to agree to One China by force, as the PRC has made it very clear that the PLA (People’s liberation Army) is invading if Taiwan abandon the stance.
With that in mind, Taiwan and China situation is really similar to a civil war, Taiwan is just a secessionist. And until the situation is resolved, there is nothing wrong with label it “Taiwan, China”