Much better photos! Thanks for going to the extra trouble.
Adafruit support has by far the most experience with such things and usually picks up on issues right away.
The only things I see (in the latest batch of photos),
there appears to be excess solder on the 'tube side' of the pc board.
And, probably not related but there may be a scratch through the trace that runs around the pc board (opposite the tube).
You can remove extra solder using solder-wick or apply extra flux and re-melt.
For the scratch in the pc board trace, carefully scrape the solder-mask to expose bright copper and bridge the gap with solder.
I am not sure which digit you indicated is #3.
Tube pin-19 appears to me to be digit-7, the un-used digit between the minutes & seconds.
philmonty wrote:P.S. Is there any sort of reference that shows the tube pin numbers as per the PCB?
From the schematic, Digit 3 (VFD3) is tube pin-16.
Good luck with the trouble-shooting.
VFD3 goes back to connector J2 pin-3.
Connector J2 pin-3 goes back to the MAX6921 pin-21.
You can keep checking back until the 30-60-v shows up.
When you find that spot, the problem is between where the v shows up and where it disappears.