It appears that this is controlled by, in my case, three keys under the HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ContentDeliveryManager branch.
the ones that start with SubscribedContent
set them all to zero.
I have done this now, but i cant make the screen pop up reliably, so i dont know if its fixed the problem. Its not popping up on second reboot of the same machine, even before i changed these keys.
Another problem, is that these are user keys. So like my digital signage runs as an unprivileged local user, so no group policy will apply to it. Have to make disabling these keys part of my setup signage script, or find another way to push them to all users.
ref:
https://www.majorgeeks.com/content/page/how_to_disable_get_even_more_out_of_windows.html
Note that i dont have the above key exactly, but three similarly named different keys. When i search those strinsg, they all have to do with **** i dont want and that microsoft can seemingly randomly push to machines. so we want those off.
reg add HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ContentDeliveryManager /v "SubscribedContent-338388Enabled" /d "0" /t REG_DWORD /f
reg add HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ContentDeliveryManager /v "SubscribedContent-338389Enabled" /d "0" /t REG_DWORD /f
reg add HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ContentDeliveryManager /v "SubscribedContent-353696Enabled" /d "0" /t REG_DWORD /f
edit: this seems to have worked. put in a computer policy for windows 10 machines, targeting the default user container. But i cant reproduce the error at will so cant test if this solves it.
computer policy for NEW users
user policy for existing users:
gotta do all 3.