The rise-then-drop is normal and common. Your aerobic system was still warming up. Then once it was, it reduced the lactate in your bloodstream. The aerobic system uses three fuels: oxygen, fat, and lactate (pyruvate).
I think you’re safe to use 155 as your AeT HR (the upper limit of Z2).
Did the test go any higher? Did it record HR at ~4 mM or MLSS?