More info: a few days ago, the problem escalated. Instead of the sensor becoming unresponsive until reboot, it occasionally crashed the phone--that is, the phone would occasionally spontaneously reboot right when I tried to use the iris scanner.
So I exchanged the phone and the new one has been fine so far: no crashes and no problems with scanning irises (or fingerprints--some people have reported similar problems with that sensor). The phone has been up 54 hours now. Hoping the good luck continues.
My different experience with two instances of the S8+ increases the likelihood that it was a hardware problem, but that's still not certain. Even if it's a software bug (perhaps in the sensor drivers), it could be that slight hardware variations (all within design parameters) affect timing enough to trigger the software bug in some phones only.