Active X controls are being converted to form controls and breaking

First of all, I would send a frown about this, this should not be happening IMO. Just open your file in Excel after it has been hit by the issue, remove all sensitive content and then click Help, Feedback, Report a problem. Check the add attachment box if you are allowed.

I know there are files which Excel-o does not allow you to open, but I am not sure what you have to add to them to make this happen. This web page mentions removing XML maps from a workbook to allow on-line editing: https://support.microsoft.com/en-us/office/edit-a-workbook-that-contains-features-unsupported-by-excel-for-the-web-7220105d-97ad-4c35-95c8-58d4bbfd14af

You could add a bogus xml map to your workbook on purpose :-)