We're thinking along the same lines. I suspect something has gone horribly wrong on the computer side. Gmail on phones uses IMAP by default, and it could be someone switched to POP, but unlikely.
If the phone is still using IMAP, I'd just nuke the account out of the email client and then set it up again anew, using IMAP. All the messages will still be there, including the ones that have not, as yet, made an appearance on the computer side.
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
It is better to be hated for what you are than to be loved for what you are not.
~ André Gide