H96Pro+ HDMI CEC "mostly" working

with my actual Panasonic TV HDMI CEC to my Amlogic S912 box is working incomplete. The basic keys (Arrow keys, OK, BACK, Numbers …) are working most of the time. Sometimes I have to switch off/on the TV to make them work.
The player control keys (PLAY, PAUSE, FF, REW, …) are not working anymore.
How to debug HDMI CEC input events?
Thanks for any hint.