In fact, measurement of Power Gain vs. Output Power is more suitable to define P1dB.Then the corresponding Output Power@G1dB is found
Such curves above are a bit confusing.
If the Gain increases with Input Power, there are some problems such as :
-Unwanted Feedback
-Temperature Drift/Thermal Issues
-Optimum Load Impedance Changes-if applicable-( Output Power is relatively low, this probability is also low )
-Measurement Errors
Can you post the circuit diagram and its layout too ??
The name of the phenomenon is, gain expansion.
The reason is only the bias point of the device. Usually appears in devices biased in Class-AB.
As was stated, just increase the quiescent current (near the class-A bias) and the gain expansion will disappear.
Yes, gain expansion is mainly observed in power amplifiers, because only the power amplifiers are biased in class-AB.
LNAs always should work in Class-A (by definition).