Can you please test if you have the same behaviour with recent upstream kernels? For instance test with 6.8.9-1 in unstable, or if you can build upstream stable version 6.8.10, 6.9.1.
6.8.9 - same behavior
6.9.1 from pristine upstream tar shows the same behavior too.
Among the new config options in 6.9.1, CONFIG_DRM_DISPLAY_DP_TUNNEL and CONFIG_DRM_I915_DP_TUNNEL got enabled. I suppose this isn't relevant.
Thank you for your attention!