Looks like this is only happening with shortcuts, e.g if I run the cmd shortcut from the start menu as admin, nothing happens, but if I actually go to
%windor%/system32/cmd.exe it works.
Basically exactly the same as this question but on Windows 10.
I tried all the given solutions but nothing worked.
This is extremely frustrating and I have always been able to run as admin before - it just suddenly stopped.