it is the same for me -- most won't install (resulting in the 'unexpected format' error) but a few install without any issues. Annoyingly, there seems to be no obvious reason for this difference I can see -- it is not related to the size of the .abr file, how many brushes it includes, or the vintage or source of the files.
The existence of this bug brings to mind two things:
It is more important than ever to make .afbrushes backups of all added brush categories that came from .abr files, because if it is ever necessary or desirable to reset the brushes to factory defaults, there is no way to get them all back until such time as this bug is resolved.
We really, really need a more user friendly & less tedious way to backup not just individual brush categories but all of the categorized items that any factory reset wipes out. If nothing else, a 'batch' backup of each categorized item that saved them to individual category files in a folder somewhere would work.