Display port MST not working in newer kernels


I have a Lenovo T495 that I connect (with a dock) to two external displays via display port. With the kernels that are available in dom0 for Qubes OS 4.1 (both current and current-testing) I can not get the two external screens to work.
If I use kernel 6.1.35 or 6.3.9 then the computer just reboots when it is supposed to show the login screen.
If I use kernel 6.4.7 then I can get to the login screen and log in, but no way for me to get any picture on the two external screens.
It seems like the fix from Displays behind MST hubs non-functional (regression in kernel 6.1) (#2171) · Issues · drm / amd · GitLab was not enough for my setup.
Any ideas about how to fix this? Or even troubleshoot this?

I don´t know if it is relevant but I have another issue which is that early in the boot then the screens freeze when I am connected to the dock. Specifically they freeze when trying to unlock the hard drive (this is the Plymouth screen). But then it works to switch to text console and back again and Plymouth is working again and I can unlock the hard drive like I normally do. This started happening after changing from EFI booting to GRUB booting which somehow seems to interfere with EDID and thus display settings, I am guessing.

But as I say, not sure if this latter issue is relevant to the first.

The first issue makes it so that I am stuck on kernel 5.15.103 and since Qubes has switched to 6.1 kernels then the 5.15 series won’t get any more updates I guess. So I really need to figure out a way to get the screens to work on the 6.X kernels.

Any help welcome.