Can confirm this to be present for a while now.
Currently occuring on
Firefox Nightly 75.0a1 (2020-03-08) (64-bit) on latest Arch Linux with wayland.
I can also confirm this
It also affects the rendering of text selection - while selecting text with the mouse on a web page, the visual highlight only appears on mouse-up, instead of throughout the mouse movement
Entering full screen media playback in this buggy state causes the initial full screen frame to display, but then no screen updates occur. Pressing Esc (exiting fullscreen) also doesn't update the screen, although it does work - using the above mentioned workaround restores normal behaviour, and updates the screen again normally.
For the fullscreen video case (last quote), my workaround is switching to another workspace and back to the current one, sometimes more than once. But almost always fixable this way.
What is needed for further diagnosis?