Just got an LG C3 and ran into same issue with an RPi4 and LibreELEC 12.0.1. I am running everything through an AVR and haven't tested if behavior is different when RPi4 is plugged directly into TV because I really don't want to connect that way in general.
After a little testing, I found that if I set the Kodi interface to 1080i then the LG doesn't go into PC mode, but when I set it to 1080p it does.
When I use 1080i for the interface and then play 1080p content, the LG still does not go into PC Mode, so it seems like something that could be specific to Kodi or LibreELEC tied to the interface? I haven't tried a bunch of different 1080p content to see if it will flip after the fact.