I'm unsure if this is because of the modpack im playing (Project Ozone 3 A New Way Forward), but I was veinmining marble with my unbreakable pickaxe and it just straight broke. It went from full durability to broken at some random point while veinmining the marble. The picture shows my pickaxe where its both unbreakable and broken at the same time.

Magically brittle sometimes just straight up breaks tools. Not a bug, and not ours besides, that's on ExU.

Thanks for clearing that up, i guess it's a pretty low chance to break as i've never had that happen before