Please show us the online values of the FB instance DB.
If the parameter GAIN has the default value of +1.0 then it would be a possible value of 0.0 for LMN as a snapshot.
Setpoint is 16.0, process valus is 20.3, so process value is too high and the PID output would decrease it's output down to LO_LMN (default value 0.0). Depends on your settings of P_SEL, I_SEL, D_DEL and GAIN, TI, TD.