bwells2 Glad that helped! I think how it's supposed to work is that by default, file transfer is disabled (i.e. usb data is enabled, but access to the filesystem is blocked and you have to explicitly enable it), but you get a notification when a data-capable connection is available, and tapping that notification takes you to the setting where you can select file transfer, tethering, webcam, etc. For some reason, the existence of a data-capable connection isn't being noticed, so the notification isn't triggering and these options are greyed out as if you were just connected to a charger. There's no data transfer at all, it seems; the phone doesn't even appear in device manager.
For some reason, changing the default forces the OS to check the connection, and it activates the data link and triggers the notification. I didn't test it, but I suspect that setting any option other than "no transfer" would probably work.
I opened an issue in the tracker (https://github.com/GrapheneOS/os-issue-tracker/issues/6641), although I kind of suspect that it's an upstream bug, given that the GOS-specific USB hardening seems to work fine.