Why do Calculation fields lose functionality when sent FOR SIGNATURE?

I've been trying to get this done for at least a year now. 

I start off with a simple set of calculation fields. (SIMPLE ADDITION)

A.

B.

This would be the desired result when sent to signature the the client would be able to place some information and have it calculated.

When A. is sent to (with all blank fields ready to receive data and the calculation field at the bottom ) SIGNATURE the calculation field reverts back to a regular TEXT INPUT FIELD. and there is no way of updating it.

C.

D. THIS IS WHAT THE CLIENT WOULD SEE.  Entering the information but NO CALCULATION...

E. Final E-SIGNED document no calculations.

was on chat for over an hour with technician.  They couldn't figure it out.