That’s a strange one,
I have the same sort of set up,
Laptop, Monitor connected via HDMI.
I have the main Cubase Artist 10.5 open on my Laptop full screen and then all other open windows go on to my external 23" monitor including the mixer in full screen view, Which I have just above my Laptop,
Never so much as a fart.
I can only ask if all drivers are up to date, Windows 10 and graphics card.
Regards Dave.