I AM NOT an aeration expert. However, I did study water body wave properties due to wind a very long time ago.

I believe you need to move your surface aeration away from the deepest water in your pond. A gentle surface disturbance THAT IS CONTINUOUS can put the water column into motion at surprisingly great depths.

Remember stirring the powdered Koolaid in your glass when you were a kid? Even if you inserted the spoon only half way into the glass, you could easily make a vortex that reached all of the way to the bottom.

I suspect that is what is happening with your pond. You have a small gyre rotating your pond water. As the deep water rotates up onto a shallow bank, it must displace the warm water, and the cool bottom water warms a little bit in that location. The deep cool water in your pond gradually warms as it makes that circuit.

Just my 2 cents worth! Good luck getting your trout back to happy conditions.