Hello tim79mc,
Thanks for reaching out to the Apple Support Communities.
So we understand the issue fully, the mouse cursor is disappearing when moving from your MacBook to your external monitor, correct?
If so, have you tried increasing the cursor size to see if that has any affect on the issue?
If not give, that a try with the steps shown here: Make the pointer easier to see on Mac - Apple Support
1. On your Mac, choose Apple menu
> System Settings, click Accessibility
in the sidebar, then click Display on the right. (You may need to scroll down.)
2. Go to Pointer, then set any of these options:
* Shake mouse pointer to locate: Turn this option on to make the pointer larger when you quickly move your finger on the trackpad or quickly move the mouse.
* Pointer size: Drag the slider until the pointer size is right for you.
* Pointer outline color: Click the color well to select a color for the pointer’s outline.
* Pointer fill color: Click the color well to select a color for the inside of the pointer.
* Reset Colors: Use the default pointer outline color (white) and fill color (black).
After making any changes to the cursor size, continue to use your MacBook as normal, and keep us updated on the results.
Take care!