I downloaded and installed Resolve 9.1 and I have noticed an oddity, when I stop on a frame the picture seems to split to 4 and pull ever so slightly to the corners. The result is softer image on freeze.
This only happens on the video monitor not on the computer screen.
I am using 720p footage and playing out both fields.
I had a similar problem. I had a project started before the upgrade to 9.1. My issue was that when I hit play the color would shift. I didn't have the problem on a new project. Try toggling some master project settings (I toggled output lut to waveform), save, reboot, and see if it helps. This worked for me.