I have found a solution, but it only works in my friend’s (fresh) Whonix 15, and not mine. The easy test anyone can do is to do
pkexec mousepad from Workstation Terminal. It SHOULD launch the pkexec prompt, as a simple test of its operation.
It does in my friend’s, but it does not in mine. This is literally the latest Whonix OVA I’m trying
Does it work in yours? Can others please test this and report back?