Hey man ,
Any constraint works the same as far as plotting them. As long as you're assigning the part of the control rig as constraint object... very important then when you plot to control rig the effect of the constraint should be plotted as well.
Also if you're in story, IF the hand is assigned to the character track correctly then you should be fine. Anything that is affecting that hand(expression constraint.) the hand should still get correctly plotted as long as it is correctly assigned in your expression constraint.
Try to separate the constraint all together and put the constraint on a constraint track and then plot them all, see if that gets included correctly so you don't loose any keys.
I personally find the expression constraint vary tedious to work with , is there any specific reason why would you use that particular constraint ?