We've seen this before with Microsoft 365 mailboxes where the mailbox gets added to a Mail Enabled Security Group and the On Premises Engine then is started shortly after.
In this case the Microsoft 365 Graph API hasn't updated the application access rights for the mailbox and SigParser gets an error which doesn't allow accessing the emails. In this case the mailbox appears to have finish even though no emails were processed.
If you wait for the On Premise Engine to run the next time this should be resolved. If you want to reset all the dates to blank again then contact our support team.