Hi Maxime,
This one should fix your issue Feel free to test it and let me know if it's not the case
I confirm that the patches fix the issue I was seeing.
I also applied the sent patches [PATCH 1/2] drm/vc4: Correct lbm size and calculation [PATCH 2/2] drm/vc4: Correct POS1_SCL for hvs5 to the 5.10.9 kernel, and the symptom disappeared on my Raspberry Pi 4B 8GB.
Thank you very much! Ryutaroh