Bob Benedict, KD8CGH
I suspected the cal also. When I repeated a sweep with the wires I used for "through" I got what I listed above - almost perfect cal for 10 MHz to 30 MHz:
VSWR 1.03, return loss -35 dB and S21 gain -.077 dB.
As stated in OP - this is my first try at measuring a filter with the NanoVNA, I would not be surprised to find a measurement error obvious to others well versed.