Here's what I found:
Output of 1.85GHz 2nd LO at input to filter: 11.99dBm
Output of 1.85GHz 2nd LO BPF:8.9dBm
Filter S21:
My filter response is a bit different than yours, I see a bit less loss, and it looks like my 2nd LO is considerably higher than what you see. Since the 2nd LO should not be affected by the 1st LO level, I'm sure my excess gain I made there has nothing to do with the higher level.
My front panel CAL control can bring the -20dBm 50MHz signal from -16dBm (full CCW) to -21.1dBm (full CW) at 1MHz span, 10KHz RBW.
Hope this helps!
73,
Matt