Spreadsheet corrupts upon saving it

After reaching out to MS Support regarding this, they suggested I post my issue over here.

Starting this week, a spreadsheet that I've been using for years is no longer working. When I launched it yesterday, it opened without issue as it usually would, I was able to make some edits and save. But after opening it up again, it came up with the prompt: "We found a problem with some content. Do you want us to try to recover as much as we can?". After clicking on Yes, comes up with "Excel was able to open the file by repairing or removing the unreadable content. Removed Part: /xl/drawings/drawing1.xml part. (Drawing shape)".

At this point, I restored a saved copy from backup (from around five days prior). Once again, file opened and was working fine, but only after saving it would it corrupt upon reopening. To make the issue even more strange, this same behavior is now retroactively occurring if I try to open a template version of this spreadsheet I had saved from 2024, or even as far as 2020. It's almost like an update was pushed which made objects used in this spreadsheet retroactively no longer viable and corrupted.

This behavior is not occurring on new spreadsheets I create, nor other ones I tested that are completely different to this specific spreadsheet I'm referencing above; though I haven't tested every single spreadsheet on my system.

I've tried repairing MS Office, I've tried saving as different extensions (i.e. from xlsm to xlsx), but no matter what I try, this spreadsheet and any version of it historically saved (and I have years worth of copies) are now all exhibiting this error if I open and save them.

Thoughts?