Following the US sneak attack (no prior diplomatic events/messages) I found myself the proud new owner of 7 transport squadrons in Quebec and Halifax. (I've yet to check all my ports around the world to see if there are others.) Would warships have been seized as well, or are just transports affected?
I thought it was a nice gesture of them (on Canada Day no less) to begin their war reparations even before I torch the White House again, but I don't think it was deliberate.

Recommendation: When declaring war a check should be made, and a warning issued before the wardec is implemented, to the AI/player if they have fleets in the war target's ports so they can put to sea before ending their turn.
Would land units using a passage treaty have the same problem?