Your measurement looks about right. I think your circuit looks like a loop antenna, of about 20 cm in circumference, so it will start to radiate well when the wavelength gets below ~ 80 cm, about 400 MHz.
The loss you see is radiation - the circuit/antenna is radiating the signal into space, not absorbing it.
Stray inductance calculations don't work when the circuit is a good fraction of a wavelength, but if you like, 60 nH is 170 $\Omega$ at 450 MHz, which is a lot more than 50 $\Omega$.
For a circuit to work at UHF, keep the wire lengths shorter than 1 or 2 cm, or better, use 50 ohm track on the circuit board. For starters, solder the SMA connectors directly to the board. You could place them on edge, or better, use a single sided board, and solder them flat on to the copper side, with their pins sticking through, ready for inventing your circuit on the clean side.
Here's a picture of a good looking DIY UHF construction method (from here)