Hi @Mads Kahl
According to your information above, OWA redirection not working for users migrated back to on-premise from online in your hybrid environment.
Please make sure if you type on-premise OWA address work for migrated users properly. Also provide any screenshot about the error you meet above here.
Exchange Online should show the redirect page for mailboxes still on or moved back on-premises.
Please try removing O365 license After migration completed.
And the official document here which introduces the steps about Move Exchange Online mailboxes to the on-premises organization in detail
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