Why does SharePoint Document folder stop syncing

We have a small number of users where their Synced libraries will suddenly stop syncing.

Background:

This is the tail end of an on-prem File Share to SharePoint migration. The migration went well. Instead of mapping to network drives they are now using the OneDrive client to sync to the SharePoint Document Libraries. Most users are fine but a handful are having an issue where the local Document Library will stop syncing. A restart of the PC or simply restarting the OneDrive application does nothing. When the Document Library stops syncing it no longer appears under the Accounts section of the OneDrive app.

One of the main Document Libraries is huge so when we kick off a resync the resync can take hours. All user machines are either Windows 10 or 11, running all latest versions of OneDrive. 

Has anyone ever come across this?