I'm also on 20.04 and I have encountered the same problem.
This problem only (but always) happens if the number of files in the folder (that is: icons in the window) exactly fit the window size.
If you resize the window you'll see that the problem disappears.
(that is also the workaround I use)
Please add your 'this bug affects me too' to the bugreport here:
And also here:
It is one of the strangest bugs I've ever encountered
but at least I can use this workaround so it bothers me not too much anymore.
