I thought this might be dirt, but once I took the joycon apart and lifted out the ribbon cable for the shoulder button and then desoldered the push button, I could see the pads had lifted (and vanished).
Then the ribbon just split entirely (with no pressure). I think perhaps it was damaged when it was closed up at the factory and just waiting to completely fail.
I did try to expose the trace on the flex PCB but it was so fragile that the trace itself quickly vanished. So I ordered a replacement (£3.50, I think I overpaid) and installed during lunch.