I don't get that problem, but I do get an issue where sometimes I don't get mail in my Inbox despite it buzzing on my watch, phone or iPad (which are all also on iOS 14, iPadOS 14 and watchOS 7). I have several Exchange accounts that worked fine under Catalina. They come through eventually, but despite several manual checks of mail they can sometimes take an hour or so. Only started happening when I installed the beta. Has anyone else had this? I've searched but can't see anything on here, though that doesn't mean it hasn't been mentioned and if so, I apologise. Obviously, I'm aware we're on Beta 1 and is working well across all my devices...not had a single springboard crash and this Mail issue is the only one I've discovered across the board. Well, that and the occasional late notification of an already read email or iMessage on my Mac.
Weirdly. I just tried to send myself a new email to test my iCloud and see if that is immune to this, and then the emails all came through. Not sure if it's a coincidence.