Yes - been suffering this for a while. You have to run the build script or the editor styles just don’t load - this is since the editor was wrapped in an iframe.
I asked about it previously here - I think this is same issue? Editor styles not loading in WP6.3 with bud dev
The workaround is to have an API v2 block enabled somewhere as this prevents the full iframe editor.
I can only hope there is a solution soon as it’s really annoying having to build the styles to see editor.css changes in the backend.