Why does the VNA show negative gain(S21) for a high power RF amplifier?
Some possibilities:

Your analyzer has low impedance, and is significantly loading the output of the amplifier.

You are connecting the two ports up wrong.  Somehow the output is being shorted or partially shorted, perhaps due to incorrect common ground connection with the analyzer.

Because of either of the above, the supply is partially collapsing, the power voltage is below spec, so of course there is no guarantee any more what the chip might do.



Check the datasheet and see if 900 mA is ever a valid current draw value.  If not, then you are probably shorting the output somehow.  If it is, then fix the power supply design so that the chip gets the right voltage over the full current range.