aup3-wal file remains, and leads to corrupt aup3

Yes, that could easily cause corruption of the project. Audacity requires exclusive access to the AUP3 file while the project is open. Unfortunately with cloud drives I don’t think it is possible to enforce exclusive access - the best we can do is to state in the manual
(Managing Audacity Projects - Audacity Manual):

You are > strongly > advised > NOT > to save your active project to an external USB stick/disk, networked storage or cloud storage