Enter f%prime(y)=func e^x via command window. Finish editing. Double click formula and now in visual edit mode exchange y with x. Notice, that an additional % is added in the command window. The command is now { f %%prime { ( { x } ) = e rsup x } } This additional % will break the formula when you save and reload the document.
Thanks Regina ! :-) the fix is in master, will be in 3.3.2, and perhaps 3.3.1 if we can get some review from Caolan (and the fix was from Luke Dixon).