It needs to be implemented per-game because of quantization. See, tonemapping operators are reversible, meaning you can feed in a tonemapped image into the inverse tonemapper and get an exact duplicate of the non-tonemapped HDR image. Well…you can provided it hasn’t been 8bit quantization. If it has then there are only 255 possible values, because it throws away everything in between as part of the process, meaning that even once you do the inverse tonemap there still only 255 possible values, only now there is a vast gulf of possible brightness between 254 and 255. As an example, maybe 254 is 300 nits, and 255 is 3000nits. Well that’s going to be noticeable, even more noticeable than the banding when just using SDR.

In order to implement HDR properly it needs to be implemented before quantization.

It’s called mura, and it’s inherent to OLED technology (for those who don’t know the O in OLED stands for organic, perfect panels are basically impossible).