Matching S22 with a metal plate at the output
If the metal plate is 'little', then it's behaving like a small capacitance.

If the connection has excess inductance, or in other words is high impedance, then this extra capacitance can form a low pass filter with the inductance to match it across the frequency range of interest.

Ask your colleagues how they knew where to add the extra capacitance. It may be that that particular assembly always needs a bit of extra C there. Or they may have measured the S22, and noticed some ripple, with a frequency that told them how far the defect was from the connector, and a phase that told them whether it was high or low impedance. They may have measured the S22 with a TDR, which basically does the sums of the last sentence for you. Or they may have been dabbing a bit of extra C around experimentally, to see if anything improved the S22 (it's not illegal and it's quick to do, I'm not ashamed to do it!)