With the Player-Settings (Processing) PRIME Render Method 'Direct To Plane' there are only black screenshots possible, no content of the current TV (with IPTV-simple). When I switch to EGL, the screenshots are like as expected and working.
Then - when "Switch to full screen" is Never under PVR & Live TV settings, the first picture on any channel selection is black. Switched to full screen the picture arrives. Works also better with EGL.