Cockpit tech: ANT+ Power meter → Android phone?
Several suggestions in the comments helped steer me in the right direction.
The suggestion from Andrew about switching off Bluetooth on the phone and seeing if the phone still 'saw' ANT+ IDs was the clincher (I already knew that it saw nothing, but Andrew's comment made me ask WHY).
It turned out that the ANT+ Radio service on the phone required an additional permission.
This Reddit thread concerns a different phone (a OnePlus 5; I have a Samsung), but it was exactly the right solution.
ANT+ Radio must be given an additional permission to use ANT hardware.
This must be a relatively new thing, because back when I had a OnePlus One, there wasn't any hardware permission required (the only ANT+ device I had back then was a cadence sensor).
When I granted that additional permission, the hub was detected by the Wahoo Android app.
I just went for a brief ride to make sure that the app was receiving power numbers: it was. The hub is detected and data is being sent to the Wahoo app.
Problem solved - thanks to all.