I Have a similar mouse pointer bug. I have a PC with 2 4k screens as Multiplicity master and a laptop with an external screen (4K) and it's internal screen (1K) as slave. The mouse pointer size is set to 2 on both systems, but as soon as I move the mouse to the laptop, the mouse size on the primary reduces to 1. When I check the pointer size settings it looks to be 2, but I have to change it to another size before it increases it's size again.
I must say, I have the 3 4K screens since 2 weeks. Before that I had 3 screens (1920x1200) but didn't use increased ponter size, so I don't know if this feature allready excisted!
Both systems run Win10 version 1909, build 18363.778
Mouse and keyboard are wired (USB), thus the original Windows software drivers 10.0.18362
I can reset the primary PC, and pointer size comes enlarged back on again. But as soon as I move the pointer to the laptop, the pointer size falls back to size 1. Strange enough, the size of the text editing pointer on this webpage remains enlarged, but the size of the arrow (the moving pointer) is reduced to size 1. Apparently there is a difference in pointer size between the arrow and the text editing (capital I in Book Antiqua font)) pointer function?