I have this weird issue where the New button is not appearing on the related list for a custom object. The profile in question has Create permissions (it's System Administrator), and when they click "View All", the resulting page has the New button. The button appears on all page layouts for the object. It also exists on the classic page, but not LEX. What else could be the issue here? This wasn't a problem until pretty recently (it was working fine before), I think it has something to do with the upgrade to Winter '21.