Just tested this on 5.3.1 and noticed the same issue with path tracing.
Image is a blurry mess with MRQ but looks great with HighResShot (same total samples, same resolution, denoiser off). DoF and motion blur are off.
Clearly HighResShot is not usable for animations (legacy movie render also seems to work fine, but is just a mess to use with the path tracer).
Disabling multisample effects in MRQ helps quite a bit, but has unintended consequences. I will try the Sharpness material workaround, but should this be reported as a bug?
As a side note: using less than 2 temporal samples breaks the light setup making the scene much darker and going all in with spatial samples crashes UE for memory usage (i.e. 8k samples in high res shot works, 1024 spatial x 8 temporal in MRQ works but 8192 spatial x 1 temporal in MRQ leads to a crash).
@ loveulb to use the Sharpness material, create a new material, pick PostProcess as domain, copy the nodes above and add it in the camera you are using in the sequencer (in PostProcess materials).