Possibly a tribe-specific thing. I saw something on Instagram which took a clip where Bryce Dallas Howard was talking about how in season 2 Din will find out things about the Mandalorians he didn’t know, things different from his way of life. This must have been on a 24-hour-lasting instagram story, I can’t find it now. But it looks like Din will be meeting other Mandalorians and he’ll be very confused when he sees them taking their helmets off.

Likely explanation is that whatever tribe or clan or whatever just took it upon themselves to be really religious about the Mandalorian creed, and that’s why it’s a rule never to remove their helmets - as their warrior/soldier identity is simply who they are.

Edit: I don’t think I can point you to any sources about the difference in Mandalorians though. I don’t think we really know anything yet, and this will all be explained soon.