2.82a, Blender.org. Tried disabling all addons and the behavior continues.
To clarify, the actual brush size changes with the F shortcut, but the numerical radius slider in the header continues to display the same value. Note how they both display 35 px in the image below despite the radius being changed drastically with F.

Its really not a major issue, I was just confused for a few minutes why mine was behaving differently than the video.