OP here again.
For what it’s worth, I’ve now tried the same ASIO code with a Lynx AES16, and it works perfectly. So it’s either the Steinberg ASIO driver, the Yamaha/Steinberg hardware, or the USB interface that’s at fault. Probably not the driver; Steinberg invented ASIO so they should know what they’re doing. But the UR824 was bought used and the computer is old. I’ll first try a different computer and see what happens.