Edit: I found a good workaround. In Fusion 360, on the send to 3D printer dialog, if instead of choosing the Cura menu item, I pick "custom" and then select the Cura app, everything works as normal again. Must be a bug in the F360 application chooser.. Thanks for your help.
The mystery is that over the last year I have made hundreds of prints with this setup and everything worked perfectly. Then all of a sudden one day, it didn't.
OK, I cross checked using my laptop. Both desktop and laptop are M1 Macs, latest OS.
On the laptop, Cura is 4.9.1. Fusion is 2.0.10813
On the desktop, Cura 4.10.0
Both versions of Cura have garbage characters in some of the dropdown menus
Here's a clue: Fusion thinks I am running Macos 10.16.0 when I am actually on 11.5.2 WTF??
Since the same thing is happening on two different machines with different Cura versions, it must be a systemic problem that is affecting other people too.
Exporting the object and dropping it on Cura from the finder works fine. Must be the Fusion 3D exporter telling Cura where to put the print. Somehow...