We used an electric heater on the floor that never cycled off, always read 71 even when the furnace or heat pump got the top of the AS to above 78.
The heat pump cycled about every few minutes after it reached a temp where we had set it, same with the furnace, but less frequently. The air at the bottom, middle and top of the AS varied by 6-9 degrees. Night time temps were upper 30s to the 50s.
With poor air circulation in the AS the air will layer. The thermostat has a 2-4 degree difference from cycle on to cycle off. Because of the poor insulation and drafts inside I figure it just cools off pretty quickly at the floor and all the heat rises. The middle layer keeps changing and making the heaters cycle frequently.
When it starts doing that, try lowering the thermostat another 2-4 degrees and I'll bet the furnace doesn't kick in right away, but when it does, it will behave as before.
The good news is, you're warm, right?