I do not use it, but I did use "start is back", maybe it broke something.@vatsake
IF you're using Winaero Tweaker...
Make sure that "Desktop and Taskbar > Disable Action Center", is NOT checked, in Winaero Tweaker.
On MY computer, that is what caused no calendar popup for me. (on DEV, beta and RTM)
If you're NOT using Winaero Tweaker, I would still look for things related to "Action Center".
This is what Winaero does, to... "Disable Action Center". Maybe your issue is related somehow.
Disable Action Center - tweak
On a side note... in Winaero Tweaker, do NOT use: Windows 11 > "Classic Taskbar"
At minimum, that breaks the START button. It might do other un-appreciated things as well, but no START button was enough for me. :)