with a CRT display there's a certain amount of image retention which makes these fast flickering shadows appear just as transparent grey, with the floor visible underneath. i thought there's shaders out there that fake the same effect for modern screens, but i can't find them...

Hi, I had a similar problem. It appears I have solved it by forcing the monitor to use HDMI input in the settings as opposed to autodetect. Hope this helps people encountering this issue in the future ;D
Cheers,
Tina^^

@Capt_MorganCreek similar codebase. The Dev of that will likely fix the issue for both when has some time to look into it. It's a RPI issue.