@Joey Hess who would have thought monitoring an offgrid freezer fridge on another continent would be so enjoyable!
Just where on/in your water is your temp probe?
First, liquid water (obviously) is subject to convection.
Second, it has relatively poor thermal conductivity: there was an old lab demo where you sunk ice (with coarse solder wire) in a test tube of water, tilted the tube and boiled the water at the top of the tube over a bunsen burner, while observing the ice remaining ice (or only v slowly melting) at the bottom.
And I have also seen a long shallow steam bath with multiple electric elements boiling fiercely, while the temp probe stupidly mounted just below them never got over about 70'C.
May be a contributory factor here?