I guess that means that troops in those territories cannot claim that any treaty has then been broken if their soldiers are treated inhumanly? Seems to cut both ways?
Not really, Ukraine is signatory to such treaties. Even worse, those are technically Ukrainian citizens, and recently DNR/LNR have been "drafting" every male resident going as far as literally chasing them and beating them on the street:
https://vimeo.com/698397206So this basically means that Ukrainian forces have to fight against their own forcibly drafted citizens, who didn't want to be in this war to begin with. They were like 10 years old in 2014 when some dude in Kremlin decided their fate. Such a horrible mess.
Then they would fall in the category of "rebel", I assume that Ukraine, like most states do have some provision in the laws against armed rebels or secessionism. A Rebel can be usually charged with "treason", "high treason", or other around the topic of effectively being someone trying to subvert the regime. Still, they would not be PoW unless there is actually a declared war.
Despite that, the treatment cannot be inhumane. They may even have more rights than an PoW.