The blur effect seems to be broken in Windows 10.
Turning Use Mica effect on menu background on and off fixes it briefly and then breaks it again, which you can see at the 20 second mark in the video.
This issue occurs in all Start11 styles and seems to be related to the Windows Transparency Effects setting in the Settings app → Personalization → Colors.
Windows 10, 21H2 (19044.5011)
Start11 v2.11
Edit: Apparently this issue seems to be caused by Nvidia driver 566.03 or later.