Screen just turns back on with no external stimulus

[Juglugs]

When I run the script, it works as expected.
However, the screen only stays off for a second before coming straight back on without me touching anything. When it does come back, it shows no external stimulus (e.g. MOUSE_MOVED) in the terminal:

I added a 1-second delay at the start of the script in case it was a keyboard bounce or something, but that doesn't work (and would still show up as a key press in the terminal screen).

When I do then apply a stimulus, the script exits properly with the correct message: