If I am following you correctly, you have on your design page the fields for post mash volume (meas pre-boil vol) and post mash gravity and when you change the volume field you are seeing a change in the mash efficiency but when you change the gravity field you are not?
In trying to replicate this, I figured out that you need to add the field for 'meas post mash gravity' to get the mash efficiency to change. The program does not know that you did not add anything to adjust the gravity between the post-mash and pre-boil, so it does not use the pre-boil gravity for calculating your mash efficiency. This is why the mash efficiency does not change on your design page when you adjust the pre-boil efficiency. This allows for the brewer to add sugars (simple, DME, LME or other) to the boil kettle without giving an erroneous calculation for mash efficiency, even if the recipe did not call for it.