Thanks, @rickkas7. I have been checking the result of Particle.publish() using the solution you helpfully offered over here
but hadn't been using that .flag(SystemSleepFlag::WAIT_CLOUD). I will add the latter and see if that makes a difference. The sensor is deployed in the field where I will leave it for a few weeks, so I'll wait to mark this as a solution till I have a chance to test the updated firmware in that same location (noting that the firmware worked without much of a hiccup at home, as often happens)! Thanks again.