Certain files don't let me change themes after a while

Hi, this is a strange one. I am using lots of custom theme colours and sometimes need to change or update them, and some files will randomly stop letting me do that for seemingly no reason.

When broken, I can do everything else, including changing colours (or fonts etc.) manually, or with conditional formatting, but I cannot change the theme itself. The menu options are there, the themes aren't greyed out, yet they cannot be applied. I can right-click on them and edit them, but it changes nothing. I can even customise the existing theme, but that takes no effect either.

All files are my own with the same permissions. Some are online, some locally stored; some are shared, some are not, the issue can occur irrespective of any of these factors, nor is it tied to the age or size of the file. When I create a new workbook it always works. Old workbooks either do or don't, and if one has been 'corrupted' it will never work again. The issue persists across all sheets within a workbook. I can manually copy the contents of each sheet into a completely new workbook and it will work again, but it can also break again in the future. I cannot break it on purpose, and even when I once noticed it occurring during editing the only thing I could do was to revert to an old version of the file, I couldn't find what specific thing caused it using undo (although it is possible that if caught quickly I could undo it, but I usually notice it a few sessions later by which point it is too late)

I am honestly baffled by this. Has anyone any idea?