... Wait, so my privateers are not supposed to get reinforcements from my own cities? Wow *facepalm* just wow, I've encountered this "bug" more often than I can count, because I thought this was normal lol... I mean, I thought it was weird at first, just didn't think it was a bug. (I thought it was like me pretending to be a neutral army helping privateers beat a common enemy on my home turf.)

Thank you for sharing a link. It gave the final clarification.

So now bug is not to get reinforcements (as it happens on screenshot two). I'll pay attention to how often it happens during the game and perhaps log it as issue if it's indeed frequent . Anyway this topic questions recieved the answers, thanks everyone!

From what I've seen, if you're attacking with "incompatible diplomatic status" (e.g. someone with whom you're at peace, or at cold war but in their territory) then privateers can't be reinforced, supposedly because otherwise the "false flag" can't work. However, if they are attacked then you can reinforce them. This is similar to how you can't attack people in their own territory, but if they attack you then you can defend yourself and wipe their army.

Not sure what about that is a bug or not, but it seems pretty systematic so it at least sounds like something that's "working as intended" for me.