This occurrence is normal.
11.9V is the spec unloaded voltage when the solenoid driver board is *not* connected. When the solenoid driver board is connected, its capacitor at C23 stores energy on that supply rail and brings up the voltage.

Your multi-meter is giving a false reading from noise it's picking up. You need a load on that supply rail to measure it properly. Hook up a 12V lamp across that test-point to ground and remeasure it.
Also check that the ground prong on your power cable has zero ohms continuity to ground in the machine and to the GND test-point on the rectifier board.

Your question has come up before:
https://pinside.com/pinball/forum/topic/bally-as-2518-54-rectifier-boardtest-point-voltages-too-hot
https://pinside.com/pinball/forum/topic/bally-as-2518-transformer-wiring
https://pinside.com/pinball/forum/topic/bally-54-rectifier-board-12-volt-problem#post-4991448

What's the chance the noise is from the ballast running the flouro tube in the backbox? If you disconnect it does it make a difference?

I haven't personally seen it (maybe because my Fluke multi-meter gives a correct reading). It seems to affect some multi-meters and not others.
An oscilloscope would probably help to see what's being picked up.

People often get a low reading on TP2 of around 160V, caused by the resistor at R3 on the rectifier board which was probably put there to limit current draw from the analog multi-meters used back in the day.

Was thinking along the lines of EMI. Can depend on how wires are running in the transformers power looms, etc. In combination with the input capacitance of that multi-meter that's giving you the false high voltage reading with no load. Try the 100k resistor as Tuukka suggested to see if it negates it.

Anyway with the circuit loaded and your other meter giving readings in range, I'm not sure there's much point trying to "fix" this as it won't be the cause of your original boot issue.

Indeed. I have a cabinet with 40 or so analog multi-meters I had to move recently only to discover my dad left batteries in all of them 10-15 years ago. Oh dear..