Not sure what changed in the system but I'm running 10.15.6 (19G2021) and suddenly it have the mouse acceleration really fast if the system is idle for a few minutes.
As soon as I move the mouse for 1 second, the acceleration does return to normal.
I have a custom acceleration set in this system like this
defaults read .GlobalPreferences com.apple.mouse.scaling 0.125
Not sure if that matters.
I also have a Logitech mouse that is controlled by their software GHub.