Intermittent black screen on external monitor connected to M1 MacBook Pro

Brand new M1 (M1 Pro, not Max) MacBook Pro, just unpacked and updated to macOS 12.0.1. I did *not* use Migration Assistant; this is a clean install.

I've connected it with a Thunderbolt 3 cable directly to a LG UltraFine external display (that I've used for nearly two years with my previous Intel-based MacBook Pro). There is no hub or other device in between. The external display has a Western Digital USB-C hard disk daisy-chained from one of its three USB-C ports.

Intermittently — every two or three minutes — the external monitor will go black for about 10 seconds. It comes back on its own. And then I'll get a complaint that "Disk not ejected properly" for the external disk. So it's a apparently Thunderbolt 3/USB-C issue, not strictly a display issue. For some reason, the entire external monitor disconnects and reconnects.

I've removed the external hard disk from the equation. I'll worry about that later. Chasing the display problem, I've tried

1. rebooting,
2. swapping for a different USB-C cable,
3. unplugging/replugging the monitor,
4. setting the laptop monitor as primary,
5. setting the external monitor as primary, and
6. mirroring the two monitors together.

Nothing seems to help. System Information shows the LG at a 60.00 Hz refresh rate. The display always comes back fine, but it really interrupts the flow of work to have the screen black out for ten seconds every couple of minutes. Other than waiting for macOS 12.1, any suggestions? Thanks!