I have an iMac and 2 external displays and I've stumbled across a weird bug with the MacOS screen sharing feature. It seems that if in the display arrangement you have it set so that the primary display (the one with the menu bar) is on the right-hand side of the arrangement then when you share your screens with another Mac that the mouse cursor is trapped on Display 1 (the left-most display), no matter which display you are viewing on the remote Mac.
If you use arrangements to move the menu bar to another display, or move arrangement of the displays so that the primary display is not on the right, then screen sharing works as expected and you can use the mouse cursor on all displays from the remote mac.
If you use arrangements to move the menu bar to another display, or move arrangement of the displays so that the primary display is not on the right, then screen sharing works as expected and you can use the mouse cursor on all displays from the remote mac.