We have recently switched the theme to GeneratePress, at a single post page, while publishing a new post, all changes settings (such as font size, color, etc) are reflected but in old posts, changes are not appearing properly.
I have checked also while disabling the cache plugin but has not been fixed yet.
Its to do with the WP Rocket unused CSS and CSS cache.
If i check the problem URL and this query string to it; ?nowprocket it will disable WP Rocket on the front end, and then it displays correctly.