The RHP is likely heating the accurite and artificially changing the 'ambient' air temp by heating the accurite. The only way to take a air temp is to measure the temperature of the air not being influenced by a heater, especially a radiant heater. I am guessing you have not taken an ambient air reading at all. The accurite should be placed where you could not tie a sting to it and touch the face of the RHP. This is line of sight.

RHPs are radiant heat panels, the emit IR radiation, this extends like light heating any solid it touches. Think of the sun or a camp fire, on a cool night sitting facing the camp fire your front gets really warm but you back is still cold. If the IR light from a RHP hits the accurite it heats the body of it up and you end up measuring the surface of the housing rather than the enclosure air. Follow?

Wikipedia, "Radiant heating, It is the method of intentionally using the principles of radiant heat to transfer radiant energy from an emitting heat source to an object. Designs with radiant heating are seen as replacements for conventional convection heating as well as a way of supplying confined outdoor heating." This is the exact same it is why radiant heaters are sold for out door patios it heats the people under it.

The ambient air temp determines how much humidity you could have, the RH is a % of how much close you are to the saturation. The numbers you have are very high too high really, if your ambient air temp is lower it starts to make more sense because the cooler air can hold less humidity and 80% of a smaller number is smaller. It varies it is actually a proper % if you pay sales tax on a car it is the same % but a different dollar amount than on a pop.

This means that if your RHP is doing what you want it to if you keep everything sealed off with it on the RH will decrease and when it is off it will increase. RHPs in my experience make little change to Humidity.

Relative Humidity:The amount of water vapor in the air at any given time is usually less than that required to saturate the air. The relative humidity is the percent of saturation humidity, generally calculated in relation to saturated vapor density.