Update. Have played with this a bit now. I can say, something has definitely changed.
Prior to my report, mouse at least was usable on external screen very reliably. (Desktop mode itself not exactly refined, of course, but at least the mouse worked every time.)
Now, since my report, mouse function on external display is extremely unreliable. Most of the time it's not moving over to the external display. Occasionally, maybe 10% of the time, it does. Very weird.
A reliable and usable desktop mode would be quite the promotion for android and graphene os. I guess not everyone wants that option to be available. As of today, it isn't. I'll continue trying to use it, when it's trustworthy. Obviously until then for regular desktop activities I'm sticking with another, more secure platform.