I have experienced exact same bug with iPadOS 14 and 14.0.1. Followed the same steps to try to fix it without success. Reported bug and also spoke to Apple Support with the only suggestion being that perhaps there was intermittent connection problem with ISP.. The bug is intermittent. It occurs regardless of used and with all accounts. My Mail accounts are all POP. I will try IMAP. This bug has also been reported several times on iPad Discussion at Apple Support so we are not alone.