I took this thread less as a bug report and more as a "Heads up, this is the behavior, before this catches anyone else off-guard like it did me." to other players. There's a place for both types of post, honestly.
I mean... I do tend to think you're correct that this is (sort of) "working as intended", inasmuch as I suspect the animals have records allocated in a different system when being cared for by the mammets, which would be left as garbage state if the animal were released external to that system; it would explain why you can't release the animal without first ending the mammet's orders to care for the animal.
And while I personally still think they could improve the messaging -- and ideally block you from attempting the capture (or at least, provide a warning) if you have no free spots -- that's not really a "bug report" so much as an "improvement request" for the UX, to make the situation a recoverable error rather than an unrecoverable one. And that -- feature requests/improvements -- is something the CS folks do direct people to post in General Discussion for... well, general discussion.
