Why can't I draw straight lines with my tablet pen?

[GIMPsuit]

No, I'm not talking about my hand technique.   

I'm referring to the straight line feature achieved with Shift+LeftClick. The feature works just fine with my mouse. With my tablet pen, however, the dragged line appears when you press Shift, and after "clicking" (pressing the pen to the tablet), the dragged line goes away (as expected) but no line is drawn.

Otherwise, the pen works okay. Even pressure works. Does anyone have a clue?

Edit: video: https://i.imgur.com/lv40OWB.gif

Specs
Arch Linux
GIMP 2.10.20
Monoprice 10594 tablet