customiser stopped working after WP5.5 update

Both sites on which I use GP have developed a fault with the customiser since updating to WP5.5

In the customiser, the moment I make any change that would affect either additional css or certain theme options (e.g. by changing a colour) these css style sheets stop being applied so the preview loses these styles. It is then not possible to see the changes that are being made. (Note: any changes made do save properly and appear on the front end, they are just not showing in the customiser preview)

Here is a video clip showing the problem: https://i.imgur.com/jBla65f.mp4

Notice how it removes the custom styling from the preview the moment I change the additional css sheet. It does the same if I certain theme options in the customiser too (e.g. background color) but not if I change font. Once the style sheet 'breaks' (unloads?) then no further changes can be previewed.

I have tried disabling most plugins, have disabled and cleared caches, and checked for dashboard errors, but can see nothing. Switching to a different theme is unhelpful because it has no custom css anyway.

If I move the additional css from the generic WP Additional CSS to your Simple CSS panel in the customiser then the customiser works fine, but I would rather have additional css (It always used to work!)