As well I'm using Curtains in Version 1.19.1.
Need to know name of that Curtains Style that you're using and Start11 V2, you are using Windows 7 or Modern Style? Also, please post some screenshots on your startpanel.
Thank you,
Basj,
Stardock Community Assistant
I'm using this curtains style: https://www.wincustomize.com/explore/curtains/347
I made some small modifcations on the skin:
Original:

Modified Skin:

For testing purpose I used the default Windows 10 Skin in Curtains and tested the Start Menu after an reboot but it doesn't change the behavior:

I'm using the modern style startmenu in Start 11:

Skin configuration (Should be default settings):

Skin individualization (only changing mica effect on / off and the transparency level. Other settings are default):

I have all the other submenus on default (Symbol Start until backup settings).
About:

Do you need more information from my side?
I don't use Explorerpatcher, UxThemePatcher or other UI changing tools.
Regards,