I've also seen on my Fujitsu U904 (3200x1800) connected over DP to the external 4k screen. I've worked around it by using Ctrl to highlight the cursor position until the cursor returns (sometimes it takes more than a couple of hours, and the only sure way to get it back is to log out and log back in — switching to terminal console or suspending and resuming does not help).
I've also seen on my Fujitsu U904 (3200x1800) connected over DP to the external 4k screen. I've worked around it by using Ctrl to highlight the cursor position until the cursor returns (sometimes it takes more than a couple of hours, and the only sure way to get it back is to log out and log back in — switching to terminal console or suspending and resuming does not help).
Will attach more info once I hit it again.