Weird thing regarding judder and stutter with or without subtitles

Regarding my problems like in this thread ( LibreELEC 9.2 beta 1 video judder and stuttering. ), I made a discovery yesterday.

My video stuttering and the frame drops with subtitles are back these days. I don't had the slightest clue why, because there was no change of any kind. There are even micro-stutters as an extra every few seconds, very similar to the 23.97 framerate problem. The subtitle drops are nearly a second long and much worse than in the old thread.

Now I realized that these things disappear, if i change the order in which I power on the devices:
If the AVR and TV are already running and I switch on the Pi: Stuttering, in 90% of the cases.
If I power on the Pi first and second the TV and or AV-R: Everything fine.

Is this possible or a coincidence?
TV is a LG UM7450 model and AV-Receiver is a Denon X3400H with PI4. No log yet, Pi3 still works flawless in every case, no matter what device I power-on first. Checked HDMI cables - no difference. What the hell is going on in my living room?

Thanks. Stay safe everybody!

Thanx very much, Klojum!

As far as I can say now, I can't see any difference, but I still have to check and report exactly...
But as I set the manual edid, I learned 2 other things:

1. In resolution settings there are all frame rates (23,97, 24, 25, 29,97, 30, 50 and 60) with 1920 and 3840.
But with 1280x720 there is only 29,97,50 and 60. The rest is missing, don't know why. So I forced Kodi to 1920 and higher and one part of stutter and judder is gone, the one without subtitles in which Kodi played 720p files with wrong frame rate. That's a start, don't you think?
As I said, I have no idea, who doesn't give my 720 with 24 or 25 Hz. Maybe Denon?

2. You can choose resolution for each file in the submenu while already playing. But when I change this, let's say I play 720p and change it to HD or UHD, the screen doesn't render it 100% but i see only a small picture in the top left corner. I don't know, if Kodi, AV-R or LG-TV is the problem. If I restart the file, Kodi remembers the new forced resolution, but only to 1920 and gives me a full-screen picture.

The third point is, that picture quality is excellent with low resolution files, if Kodi is scaling from 720 to 1080 and the TV does the rest from 1080 to 2160! Even better than if the Denon (3400) does all the upscaling or the TV does it alone. I have nor idea why this can be...but for all the people in Yurop, who often still have TV or recordings in e.g. 720 - try it out!

Thanks, I am testing right now....

No change with the edid file form AV-R. I tried the edid data of the TV, connected directly to the TV and or the AV-R..doesn't work either.

While manually forcing edid.dat files, I realized another problem. The max resolution my RasPi scales seems to be 1920x1080. All above ends in a small unscaled picture on the TV. If scaling is done by another device, everything is correct. Please see attached picture (sorry dark, forgot the flash on my phone). The unscaled picture relates to the GUI and video, no matter which resolutions are allowed and other settings. 4K=1 allowed in config.txt.
Any ideas?