So if you're on PC and you go to "Graphics Settings", and then scroll down to "Upscaling" you have a lot of options to choose from. The problem is there is no option to turn it off and run the game at its native resolution.
What the upscaling does in Tekken 8 (and other games) is that internally it is rendering the game at a low resolution and then it upscales it to a higher resolution. So for example, the game could be rendering at a resolution of 540p and then uses an upscaler like Nvidia DLSS or AMD FSR to output that image at 4K or 1080p (whatever resolution you have set in the settings). These upscalers are essentially programs that are "guessing" on how to make that lower resolution look the same at a higher resolution.
The benefit of this is that it is far less demanding. This is probably why its forced on in Tekken 8 to allow lower spec PCs to render the game without having an impact on the netcode. (DOA6, conversely, lowers the resolution on the fly in online matches if it determines your PC cannot render it at your setting to try to ensure a smoother online experience for both players)
The downside is certain upscaling methods can give you visual artifacts like awful hair dithering. Usually, upscalers are better used in fast motion applications because upscalers are "blurring" the image in a way to get a higher resolution output. You won't notice it as much there especially with the use of motion blur. But if you are looking at static images with motion blur turned off, you can see some ugly artifacting.
You can test this yourself in the game and you should immediately notice a difference in how hair looks.
I want the option to turn off upscaling because I have a beefy enough PC to render the game at 4K@60FPS natively (that is assuming there isn't some gross unoptimization elsewhere in the game)
This video also covers the variety of upscaling options in Tekken 8 and worth checking out. It is recommended by this video to use TSR as that is Unreal Engine's own upscaling method. I've time-stamped it where they recommend TSR, but feel free to rewind and see how the other upscalers look.