I just noticed this today and restarting Muse didn't fix it: on the "start" screen, thumbnails of recent Muse files and even the generic icon for a site file are stretched in row 2. If I scroll down, the stretching resolves but other ones stretch. It seems like it happens at a certain distance from the top.
When I scrolled down little:
It doesn't affect function in that I can click to open a site but it's pretty trippy and not what the developers intended, I'm sure.
That is bizarre. I haven't seen that before.
Can you check the system requirements for your computer? Muse system requirements
Also try these
Reset preferences: Muse preferences corruption
If nothing else works, try doing a clean install. Uninstall just Adobe Muse using Use the Creative Cloud Cleaner Tool to solve installation problems and then reinstall the product.