There is a dip/bump in the AM/AM curve of the fundamental tone of AMP_B.
The dip/bump is due to the 3rd and 5th order terms of the polynomial used to model the nonlinearity. AMP_B uses a 5th order polynomial along with a quadratic smoothing of the input signal level to model the nonlinearity. The 5th order polynomial provides the gain, P1dB, IP3 and IP2 effects, while the quadratic smoothing is used to smooth the transition from P1dB to saturation. The quadratic smoothing is applied between P1dB and the output saturation level, which is set by default to approximately 3 dB above the output P1dB level. When IP3 is fairly close to 10 dB above P1dB, the fundamental tone has dip-free compression because the 5th order polynomial term does not get very strong. However, as IP3 is moved further from P1dB, both the 3rd and 5th order polynomial terms have a stronger effect on the fundamental, producing the dip as their power approaches that of the tone, then a bump as they overpower the tone.