Hi Ashwin,
Below is my understanding of the same:
When we create a Customer Master, if Mandatory function has been updated in Partner determination procedure for that Account Group (for Example SP SH BP PY) system will create those Partner Functions automatically.
In Addition to that if you have assigned a new Partner Function (For Example GP) to Account Group, system will consider that Customer as GP partner function as well. Even though it is not assigned in Partner Functions of Account group.
I will still request you to test this scenario and share your observations will all.
Regards,
MJ.