Builder applies styles from a USS file that is not actually loaded.
I’d expect to have styles defined in a USS file applied to the document currently being edited only when that USS has actually been added to the stylesheet tab.
I don’t think it comes from there. I just renamed the default theme (the one that got automatically imported), but I haven’t put anything in it yet as you can see:
I’m even more sure since I solved it by putting more specific names in the other files. Which I guess is actually the recommended way to do it, docs recommend using BEM. That would make it harder to have this issue pop up