Okay, after trying to remedy my build and testing again, I've got exactly the same incorrect results as before. The red wire from D1 rises above 300V then drops down to 145V. The D6 wire reads normally, over 300V then dropping to 222V. (Also retested Kreg voltages, again around 4.7 on the A side, and 1.87 on the B side. On A, it takes quite a time to rise all the way to 4.7, over a minute I think, while on B, it reaches 1.87 quickly and stays very steady on exactly that value.)
The 4 LEDs light, and the tube appears to be glowing normally.
Here's what I've done in the last couple of days before this retest:
Removed the shunt regulator board and inspected the underside for poor soldering or short circuits. Also checked any soldering on the top of the board.
Checked each component on the board to make sure the right components were installed. Checked all resistor values with the multimeter.
Unsoldered the shunt regulator board and started again with the wire connections - from the PC power supply board, and for each terminal pin on the D terminal.
I did not resolder at the C, B, or A terminals, but I did visually reinspect them and everything seems to be where it should be and correctly soldered.
I replaced the shielded twisted pair from the power supply board to D terminal, in case the original one was damaged when I stripped the ends.
Mainly what I've tried to do is go step by step from the power supply board to the shunt board to the D terminal, to make sure that all solder connections are sound, all wires and resistors are in the right place, and there are no short circuits.
What should I be looking at next as a possible cause of this problem? Perhaps I overheated a transistor on the shunt board?