It is a recurring issue. Almost exclusively because antivirus programs are allowed access to the profile folder and hold files for scanning while regular updates are occurring, other less common causes are the mail being stored in a cloud synced location and streaming backups. Personally I had this issue until I placed an exception in my antivirus some 10 years ago. I think it was AVG in those days. But all anti virus programs can cause the issue.

These days I don't need such a thing, as defender does not appear to suffer from the same issues.

Thanks. I also use Defender and no AVG or cloud syncing stuff, so not that. But I can soldier on using the message filter.