There are two issues around this topic, which superficially look like the same, but in fact differ quite a bit:
1) At least debatable (and also logged by Paradox) is that you "inherit" prisoners your allies/third parties haven taken in war, when you annex the home nation of those PoWs later:
Description of problem Prisoners Abroad non existing What platform are you playing on? Steam What game version are you running? 1.4.1 Please select any/all DLC you have enabled. Hellenistic world flavor pack, Epirus flavor pack...
forum.paradoxplaza.com
2) Buggy for sure is that you can get a stability hit for characters which are either in your own prison or even ones you have already released. I was able to clearly reproduce this for the case that you take a general/admiral as PoW, then annex his home country, imprison the elites - if you release that character, it gets mirrored in the list and causes a stab hit:
Description of problem Character in country considered imprisoned abroad What game version are you running? 2.0.1 What platform are you playing on? Steam Please select any/all DLC you have enabled. (DID NOT ANSWER QUESTION) Do you...
forum.paradoxplaza.com