Thanks for the input everyone.
Regarding the solder quantity, I ran out of my favourite solder and used some other solder I had which ran a lot faster than I'm used to, but thankfully that wasn't the issue.
Thanks to the comments about the resistor readings changing when in or out of the circuit, I was able to isolate the issue to the C side resistor on the -3dB contact. The issue was a poor solder joint where one resistor lead on the terminal was soldered properly, but the one in question had a void around it. I reflowed the joint and all is well!
Thanks as always for the advice and knowledge!