This disables an essential HDMI 2.0 feature. Also, the issue seem to be specific to Gemini Lake. I couldn't reproduce it with Alder Lake/Tiger Lake.
Thanks for your help on this.
Do you think it would be useful if I open a bug in the i915 kernel repo (https://gitlab.freedesktop.org/drm/i915/kernel/-/issues)? This product is probably EOL, so I wouldn't expect much progress from intel engineers, but maybe someone there might have an idea about this.