As my TV is pretty good at upscaling and lightning fast at modeswitching I decided to allow all resolutions in Kodi. That means there are many modeswitches but every time the GPU hangs for a moment showing garbage on screen. One time it even hung with a black screen for 10 seconds and another time the black screen stayed untill I rebooted (via SSH).
//EDIT: This is on ng.
//EDIT²: GPU hangs for seconds (audio plays fine) when switching from 4K SDR (kodi UI) to 4k HDR (movie) and vice-versa. This is easily recreateable.
//EDIT³: Had a talk with the famous Neil Armstrong. According to him the blob and Kodi (he tested with GBM and Wayland backend) support atomic modesetting. So this is indeed a CoreELEC specific bug!