Hi Dan, I think I may be having the same problem and I am just unsure how to apply your fix.
I am using a slider control to control the scale of an object on the x axis. The comp where the asset being controlled lives is not the same length as the main comp. The expression works great when there are no keyframes. As soon as I add keyframes to animate the scale the expression no longer updates the value of the slider.
temp = comp("Compliance Bulletin").layer("Time Left").effect("Time Left Control")("Slider");
[temp, 100];