Just encountered the same thing in my PiNet middle school lab. I have 16 installed Pi3's, all with SenseHats. Temp readings are everywhere from 320C to -45C on different Pi's.
I bought these last spring, but this is the first time I've played with the temp sensors. I'm hoping that I can get these replaced.....
Unfortunate, since it's an amazing HAT with so much possibility. This, however, will negate its usefulness for data collection and visualization.