As of today, I *HAD* the same problem on my LG G4.
After a lot of experimenting I came to the conclusion some signal is telling the HDMI port of the TV to jump into PC mode. Even the GUI at 1920x1080@60 jumped into PC mode.
Then I tried @50: no PC mode. tried @30: no PC mode. So started the whitelist with only 3840x2160@30: Happy times, no more PC mode!
Added all the other modes one by one and tested. In my case, the 3840x2160@60 and [email protected] modes on the whitelist were the culprit. Should be the same for 1920x1080 but didn't tried that yet...
Good luck with this 'typical feature' of your LG OLED TV.