This is most likely because you’re using World Position, which has some issues when used in a Post Process material with upscaling that you will need to work around, there are some different solutions in this thread: Post Process Absolute World Position is weird
If you can’t change the Blendable Location to something before upscaling, then this is the approach you’ll probably want to use: Post Process Absolute World Position is weird - #5 by Vioxtar