This is a bit of a weird edge case, but after changing one of my sites to this library after the current one was no longer viable, I noticed that only a tag with a dash or multiple dashes inside won't get processed.
What works:
[b]a[/b]
[b]a-[/b]
[b]-a[/b]
What doesn't work:
It's a bit of an edge case though, not very common thing to do but a user did this :)