I have been experiencing this bug for about 6 months now. i still dont know exactly what causes it, but I can only fix it with selecting hand tool and then back to selection tool.
Right now Iam on Macbook Pro 2017 Glyphs 3.1.1 (3146), but this have been an issue for me for a long time.
I can replicate it also on my iMac 2020.
Not sure how to replicate it but I’ve been experiencing this too.
Can you start Glyphs without plugins? Hold down the Option and Shift key when opening Glyphs.
Looks like you have the Ctrl key pressed? Is there a Bluetooth keyboard connected? Something similar once happened to me until I figured out that the keyboard in the bag was on and the key got squeezed.