As soon as the Viewport changes, Via playtest, running, and anything else that activates RunService:Run(), after stopping it breaks.
The only fix is to leave the file and go to the top left and click “FILE” and load your recent save file that you just left. For me i found no fix, Changing rendering engine did not fix it either. Same problem as the post, Also having 2 windows such as Explorer and Propertites in the same dock gui causes this for me. Putting each window in its own spot fixed my blank screen problem. I started having this when i add a “ClientAppSettings” to my Studio.