So perhaps better contact your manufacturer (sony) in the meantime. May be they will fix this via a firmware update. On my Onkyo 616 and Odroid C2 it works flawlessly.
Do you have another HDMI Device to test it with? Lets say a FireTV or a Chromecast or Bluray player? If this happens with the other devices as well, than LibreELEC team cannot change it, as the issue is on sonys side then
Who knows, may be the HDMI board on your Sony is going to break down soon or so and this is the first sign of it.
Ok, this was a good idea. I tried it with a pi running recalbox and it is the same. So moving to Sony
But thanks for helping