The size and fps display in the preview window is blinking.
What are the specs of your system?
MacBook Pro (16-inch, 2019)
2.4 GHz 8core Intel Core i9
32GB 2667 MHz DDR4
AMD Radeon Pro 5500M 8 GB
My iMac Pro has the same symptoms.
The picture above is the initial state with no settings.
This is what it’s supposed to look like. It’s live counting frames. If you want to disable it you can turn it off in the preview window options. You’re computer isn’t producing progressive frame rates, they are variable, and so it makes sense that the frame rate would flicker a different times due to your refresh rate.