External Monitor has low refresh rate on Thunderbolt 3 Dock (Sporadicly fixable)

Help
1

I recently bought a new laptop and I am having trouble connecting an ultrawide monitor with fullRes@120Hz

System Specs

Laptop: Asus Zephyrus G16 GU605MY with a 
Graphics Card: 4090 GTX (Laptop)
Port: Thunderbolt 4

Nix-Info
system: "x86_64-linux", 
multi-user?: yes, 
version: nix-env (Nix) 2.18.5, nixpkgs: /nix/store/bd4fmzws6n5542khxbifbkr6nrygi232-source

boltctl info
 ● Belkin International, Inc. Thunderbolt 3 Dock - Pro
   ├─ type:          peripheral
   ├─ name:          Thunderbolt 3 Dock - Pro
   ├─ vendor:        Belkin International, Inc.
   ├─ uuid:          c4010000-0080-8c1e-03d5-bc288c809025
   ├─ generation:    Thunderbolt 3
   ├─ status:        authorized
   │  ├─ domain:     2cfd8780-601a-0079-ffff-ffffffffffff
   │  ├─ rx speed:   40 Gb/s = 2 lanes * 20 Gb/s
   │  ├─ tx speed:   40 Gb/s = 2 lanes * 20 Gb/s
   │  └─ authflags:  none
   ├─ authorized:    Do 05 Sep 2024 13:24:05 UTC
   ├─ connected:     Do 05 Sep 2024 13:24:05 UTC
   └─ stored:        Do 05 Sep 2024 13:24:05 UTC
      ├─ policy:     iommu
      └─ key:        no

The problem only occurs when using the Thunderbolt 3 Dock. If I use a dedicated USB-> DisplayPort cable it works as expected. With the Dock

  • Full Res 120Hz works at lower resolutions
  • the FullRes@100Hz and FullRes@120Hz are being detected and shown in xrandr but lead to black screen output.
  • journalctl -u display-manager.service does not produce anything suspicious
  • FullRes@120Hz works in Windows with the DualBoot

There Also is an entry on the Asus-linux.org’s FAQ with the question

Question: Using an external Display has low FPS or is extremely laggy



Using an external display has low FPS or is extremely laggy
This problem mostly appears when connect your external display to the dGPU in Hybrid mode (rendering screens on separate GPUs). There are various reasons that can cause this problem and we don't have a perfect solution for it yet. But here are some possible ways to solve it.
Use X11 instead of Wayland.

If your model supports video via USB-C and one of the USB-C ports takes the signal from iGPU, then buy a USB-C to HDMI/DP cable or dongle. this way, you can avoid rendering two screens on separate GPUs. (tested on 2022 Zephyrus G15)

However I am using X11 and the issue persists not only in Hybrid mode but also in AsusMuxdGpu. The same configuration used to work fine with my previous laptop at 120Hz and the same dock.

BUT!!
I managed to get it work. Unfortunately only sporadicly. When running
boltctl forget <dock-id>
disconnecting and reconnecting the dock after a few seconds the display works as expected at 120Hz and the screen does not turn black. However a reboot will cause the problem to reappear. That fix does only work when the external monitor is daisy chained via thunderbolt. Using the docks display port does not work.
Is there anything I could use from this and do further investigation

Now I also recently created another post with that problem but with the findings it seems like this is not up to date anymore because the issue seems only to surface when theres a dock between display and Laptop Cannot Change Refresh Rate to more than 60Hz