After a few days, it seems like adding the 22k resistor between pins 1-2 of R4A did the trick. Haven't had any more issues with the #8 button.
Some things to note:
R4 is a 15k sip. That's what was in the kit. So with the 22k in parallel, that makes 8.9k ohms.
The transistors Q2, Q3 are FNJ3305 and Q5 is FNJ4303. This matches the schematic.
