The more I look into this, the more I think neither the RFLKT or the Fitness app is the problem. I'm using a Samsung Galaxy S5 Active, and googling produces some "authoritative" comments saying that the S5 actually discontinued the temperature sensor that was present in previous generations of the phone. You can also find posts saying the opposite, but it makes sense to me that the Fitness app might be querying a sensor that, on my phone, is just not there.