Hmm you're using a very very big sample radius there so it doesn't even look like AO anymore.
Because MXAO samples the environment in a 2D spiral, some samples may lie outside of screen boundaries. As the border pixels repeat themselves for reads outside screen, you get "false" occlusion by objects that are actually not there but are constructed by repeated depth buffer info + the coords I'm reading. As the data of the border pixels changes quickly, you can get some flickering by these false occluders. There are modes that change this behaviour for out of boundary reads but none of these help here.
3 possible solutions, 4 actually: 1) clamp sample positions to screen, this causes flickering at borders. 2) skip AO sampling for samples outside screen, this was legacy mode "boundary check" but it's too slow. 3) Don't expect anyone to use such crazy radii, that's what I opted for. 4) Use a guard band for the depth buffer that#s larger than screen, that's what most AO implementations with full engine integration do, such as Alchemy AO. They recommend a 20% guard band. As the depth buffer of ReShade matches the color buffer in size, this is all I can do, sorry.
My tip: use AO sample ranges where AO actually looks like AO.