IMHO multi track quantising IS buggy.
Issues I regularly get include: slices where there is no transient (on any of the tracks); sometimes the full Quantise panel does not open even though the folder track = was checked before recording; trimming one of the tracks in the folder group sometimes doesn’t trim all tracks in the folder; erratic behaviour when there are several takes overlaying (I now remove all takes except the one I’m working on).
It’s disappointing that after waiting years for this tool it is less than perfect. Life’s like that I guess.