I'm confirming this same issue with a nearly identical setup and attempted fixes.
Issue occurs on :
- Started with recent update to High Sierra 10.13.4 (as soon as I restarted for the update the problem was immediate)
- MacBook Pro 13-inch 2017, Two Thunderbolt 3 ports
- External monitor :
- connected through USB-C > HDMI
- 1920*1200 @60Hz (Asus PA248q)
- rotation : 90°
Like Pierre, increasing the mouse size via the Accessibility window in system preferences is a temporary solution but as soon as you scale the mouse to low it will disappear again. I tried rotating my monitor display to "standard" and can confirm that allows you to view the mouse on the external monitor at the mouse's normal size. Again, changing back to 90 degrees rotation will make it disappear again. Mirroring displays works until you go into a finder window then it disappears again. Resetting SMC and PRAM doesn't it solve it either.
Until a solution is found I will have to live with a larger cursor size. It's the only work around that I know of.
Thanks Pierre for being thorough