As I have already written, I had already tried various settings in the CEC menu. Unfortunately, these did not have the desired effect.
I could deactivate CEC once for test purposes, but switching it off permanently is not an option.
The RPI is permanently on, so it is not pinging the TV by rebooting Kodi.
It's not related to CEC. Your TV detects RPi as A/V source, and that's the cause of the switch.
If that's the case, I'll have to live with it.
Thanks for the feedback anyway 
I have been struggling with this for several years now.
As soon as I restart Libreelec on my RPI4, the TV always automatically jumps to the HDMI channel.
I have already set the following option in config.txt:
hdmi_ignore_cec_init=1
Unfortunately, this did not solve the problem either.
Adjusting the CEC options in the Kodi menu did not solve the problem either.
However, I don't want to disable CEC completely as I want to control Kodi with the TV remote control.
I would like to buy an RPi4. But I wonder if LibreElec would benefit from a Pi with more than 1GB RAM?
Might help if you mentioned the platform you are using and perhaps post a debug log.
I've not seen others reporting this not working.
I am using the Generec Build. On a self made htcp based on intel cpu.
I will try to get the logs and post them here.
So same behavior like in the OpenELEC and Kodi Forums.
No one has a clue how to solve this problem 
I was on OpenElec now I am on v7.0.2. I used the direct update to LibreELEC.
Since ever I am not able to get Pause during Framerate Change to work.
I have enabled the feature under video with a 10sec pause. But while starting videos or series the playback is not paused.
Instead the video/episode is just starting with a 1sec long pause.
This has never worked for me on OpenELEC. So it would be great if it could be solved on LibreELEC 
