tmp files do not sync but are not restricted
Originally .tmp files were restricted and not allowed to sync from Windows OneDrive client to OneDrive Online.
However, they lifted this restriction and allow all files types now according to: https://support.office.com/en-us/article/types-of-files-that-cannot-be-added-to-a-list-or-library-30be234d-e551-4c2a-8de8-f8546ffbf5b3
However, .tmp files on all our Windows machines show that they are syncing but never actually sync. It is an easy test to see that it shows any tmp file will never sync and never attempts to sync, but shows in Windows Explorer that it is in a sync state. This is a bug and the support rep from Microsoft said to add it here.
Davide Fasolo commented
We have the same problem!