Instrument sounds change after activating 2nd project

This bug is driving me crazy!
After I open a second project, activate it, and then go back and activate the first project again, the first project plays the wrong instruments. The inspector panel (on the left) shows the wrong instruments - I don’t know why they randomly change.
It ALWAYS happens when I re-activate the original project, it’s not an intermittent bug.
It reverts to the correct instruments if I “revert to saved” but it’s a big pain.
Any help greatly appreciated!

Cubase Pro Version 10.0.60 Build 396
MacBook Pro 10.13.6

Are you sure you activated it?

If yes, check if there’s a prefs problem by starting Cubase in Safe Mode

Yes, I am sure I activated it.

Is safe mode an option for 10.0.6 on a mac?
The manual for 10.5 says
“To open the Safe Mode dialog, launch Cubase, and hold down Ctrl/Cmd-Shift-Alt.”
but the accompanying graphic doesn’t include the Alt. button. And I’m using a Mac. Either way it’s not launching Cubase in a different way.