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)
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!)