I’ve noticed the same issue. It works fine with the index template, but there seem to be problems with other templates. Initially, it functions correctly, but when you save the page and reload it, or change the template from the page settings, the global post content class isn’t always loaded. As a result, any added relative styling is missing. This causes a styling difference between the frontend and backend.
I’m still experiencing the issue on 6.3, but it seems to work on my 6.4.1 staging site (just waiting for a different bug to get fixed before updating).
with 6.4.1 it is working perfectly now,
once it happen did not work in accident,
i will turn on/off the global class green/red dot, this can help for me.
I am using WordPress 6.4.1, and the issue is still there. At first, it seemed random, but now I have noticed an exact pattern: when I create a page, it works fine, and it also works fine after a page reload. However, as soon as I adjust the page template settings and select any template other than the default template, the problem occurs.
I’m not sure when exactly this regressed, but it seems to be a bug again? Global classes on the Post Content element aren’t being used in the backend DOM.
EDIT: Actually, looks like it does get applied if you Force iFrame.