i don't know how to explain this well but
when you go to the end of an item and zoom in hard enough , you end up seeing bits of the notes which are at the beginning of the item . When you select them and hit quantize , you would expect them to get nudged to the next grid but that does not happen and if you select and delete these bits of notes, the notes from at the beginning of the item get deleted .. just trying to put a gif along to see you can see whats happening
regards
zook