I’m not certain that it’s still the case but several years ago Outlook for Mac was incapable of handling certain aspects of calendars in public folders shared groups and there was some difficulty with delegation send as.

At the time the best answer I had was for the Mac users to use Outlook as much as possible and then log into webmail when they needed to send us. It’s been a few years so I can’t help but think it’s been fixed by now. Or the very least equally broken on PC.