Where is the thermostat probe and what kind of thermostat and pad is it? Does your thermostat indicate temps?
The hi just means (in all of mine) that it is 90% and higher. I keep most of mine in the basement with windows open and because were having rain now they are all HI. But 60-70% any other day...all sheds are perfect
Are you sure he just wasnt yawning? I understand snakes like feeding schedules but other than eyeballing every movement and peaking their heads out of their hide I never saw any of mine open their mouths because they wanted food.

Once I had the flexwatt go sky high and they were all on top of their hides trying to escape the heat. Luckily or they would have been cooked!