Yeah, you were quite unlucky. [] is part of the glob pattern matching … https://en.wikipedia.org/wiki/Glob_(programming) … so you were excluding all files with F, B or ! in the path.
I don’t find this usefull, so the best option might be to simply remove [], but at this point some people might rely on that feature… but yeah, it should be documented.
The way to escape them is to wrap them in [], so ;*/:[[]FB![]]:/* (the path separator gets handled automatically. This pattern is untested!).