When pressing the 2 shift buttons in FP16 you enter midi mode.
The FP is then no longer a surface control, but a midi controller .
The thing is : faders don't send midi CC in logic (only buttons ) unless you do "ignore all surface control". Strange behaviour ...
You shouldn't be obliged to erase your control surface config in logic, for midi mode to work .
As to take advantage of this function, you should be able to come and go flawlessly from surface control mode to midi mode in FP16.