I still cant believe i finally find a person have an issue which are i already have. I have a 6900hx and 3070 ti rog strix i cant find a way to enable resizable bar to enable it its just not showing on nvidia control panel on anytime mux switch active or not.(my laptop has a mux switch too) it cant be enabled, nvidia and asus responded me and they said nothing wrong with my laptop. Also i cant figure out why my pci-e working on x8 (according to gpu-z). I am checking asus website everyday for an bios update maybe it will solve my problems but i dont have a solution for our problem. But you know its happening on my laptop too.
It seems that Resizable bar is only enabled if you disable the integrated Graphis, so enabling the MUX switch. If you do this you should be able to go to Nvidia Control Panal -> Help -> System Information -> In the list to the right it says Resizeable Bar - Yes.
For ryzen the maximum PCI-e is x8, but it is gen 4 so it is more then enough bandwith for the 3070ti, so there is actually no problems here.