Calculation of a Calculated Custom Field Is Not Transferred When Pulling the Field from Field Library
When you pull out the calculated field from the library, the calculation doesn't transfer, you will have to copy paste the calculation from the other field. 

The logic behind this is that the same field in different forms can be used with different calculations. i.e. calculations are unique for each custom form. Field name, type, instructions are transferred but not the calculation. And also for this reason the Field Settings interface is split in 2 parts

The part above is affects the field everywhere and the other is only for the current form.