Hi Jordy,
We appreciate that you posted back and confirmed the detailed scenario with us.
We read your description, and we found that you created the form via Forms for Excel and you encountered the sync issue while opening the Excel file, and you don’t want to sync the data to new workbook.
If my understanding is right, for the situation you encountered, we do understand the inconvenience caused and apologize for it.
We have also tested it on our side, and we cannot reproduce the same situation as yours. Generally, when you encountered such issue, you can choose to sync to new workbook and it may help solve this issue.
However, we noticed that you don’t want to sync to new workbook, and we cannot reproduce the same situation, we assumed this issue may be related to connection between this workbook and form, and it may need to be checked from background. The best way to troubleshoot the issue is to report the issue to the related Team. We’d suggest you contact your office 365 administrator to go to Office 365 Admin Center-> Support-> New Service Request, the support engineers there have the correct escalation channel and this is the most efficient way to report such issue. You can refer to this article to check the way how to raise a ticket. Ways to contact support for business products - Admin help.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Best Regards,
Sukie