Although I think critical misses are just supposed to be misses...if you're critical miss did damage to your own party, wouldn't that make sense?
That said, it does sound like a bug. I've not had a critical miss do anything extra to me on a full playthrough.
Attack rolls are an abstraction for good vs bad outcome. They are not meant to be a simulation of projectile trajectories in a sense of it does not hit the original target but hit another target instead which is by accident even more disasterous
. If my firebolt critically missed then it's totally ok to cause some friendly fire. If enemy firebolt critically missed me, then it should cause no harm to me while possibly harm the enemy. Vice versa for critital hits. In tabletop DnD, this is supervised by DM.
EDIT: In previous post i described situation when enemy firebolt critically missed me.