you asked why it happens so often, I provided a possible explanation.
just yesterday we had a similar case where a usb ethernet adapter wouldn't work on a locked device due to a similar issue, even if that one may be more logical.
especially when you have to follow an outdated password policy where people have to change their passwords at regular intervals you'll have such cases more frequently than when they only need to set it once until a suspected compromise.
you can't delete comments from modlog, except for admins purging then, and then there is a purge modlog entry. purging also only applies to the local instance. the reason that you don't see it in modlog is that banning a user while selecting to also remove their content is only going to put the ban in modlog currently, so the comment removal was never there in the first place.
because printers are evil