High HR running downhill, HR drift on long workouts

My AeT HR is about 154.

When I run downhill, it is hard to keep below 154. Running downhill at 165HR feels easy, I can still nose breathe. Is it possible my AeT is higher for downhill running? Doesn’t make sense to me.

I think my form is pretty good, but I’m sure I can get it better.

On long days in the mountains with heat and dehydration, my HR goes higher. Again, after a few hours 165 feels more like 154. If trying to do Z2 training, should I keep at 154 and below, or does AeT change on a given day with dehydration?
After many hours, I feel like I can maintain the 165HR effort.

Thank you,