Some people do not often use reply-all when they should. My experience is this is common when people reply to emails from mobile devices, which normally have a default setting (either reply-all or direct reply).
I normally just re-add people in this situation, unless it's not clear if the individual email was meant for me specifically.
The context of each email unfortunately will dictate whether it was or not.
What I would recommend here is mentioning it to your boss casually sometime and asking if they find it rude. You could reply-all by adding people, then ask them in the future, "Hey, kind of random question - a few days/weeks ago, you had replied directly to me in a reply-all chain. I re-added people since my reply seemed relevant to them but was thinking afterwards this might seem rude, what are your thoughts?" or something similar.
You say your firm is "more on the relaxed side" which suggests this to be a quick, "nah, no problems" type of answer from your boss. But hearing your boss say this instead of random internet people would probably give you a lot more peace about the situation.