That could be it...I remember setting my TV with my PS4 Pro. Just remembered that every single inputs have their own settings....
The harder OLED is driven, the faster the pixels will wear out and lose their brightness. Since burn in is caused by uneven aging of pixels, a really high peak luminance from the OLED setting could've been a part of the problem. The 2017 and later LG models have a way of compensating for this to a small degree (in addition to other anti-burn in features), but unfortunately the 2016's seem to be the ones most susceptible to burn in.
While OLED Light should be left at 100 for HDR10 (and Dolby Vision at 50 for models prior to 2019) in order for it to be displayed properly, it should be set much lower for SDR. In properly mastered HDR, it's usually only the highlights that get really bright anyway, while the rest of the picture is comparable in brightness to SDR (there are exceptions to this of course). The main issue with raising OLED Light in SDR, however, is that the overall brightness of the image is usually increased along with the highlights. It's usually a good rule of thumb to keep OLED Light below at least 50 in SDR with LG's OLEDs.
LG has also stated that burn in can happen due to defective panels. Personally, I suspected that explanation was mostly bullshit as LG was implying it was less due to the characteristics of the display tech and more due to a whole new category in which you can lose in the panel lottery. Even still, I suppose it's possible there could be some variance in susceptibility to burn in between panels.
Regardless, I hope LG will still try to fix your panel. While Best Buy's extended warranty plan is the only one that I know of that explicitly states it covers burn in, LG seems to be replacing burned in panels more often than not here lately.