Pin 5 will stay high so long as the MPU board is operating correctly and the blanking signal is activating appropriately. If the MPU board freezes resulting in the blanking signal stopping, U19 pin 5 will go low to make sure the displays blank out and don't burn.

I'm afraid you're likely looking at a bad glass display. Those Beckman display panels used on the Stern displays suffer from this more than the Cherry display panels Bally used.
Sometimes it's caused by conductive crud shorting between pins entering the glass panel. Most of the neighboring pins will measure about 600k ohms between each other so you can check it on the bench with your multi-meter. If you get some with much lower resistance readings you may have shorts. In the right circumstances they can be removed but it's difficult and requires patience.

have you checked the rear of the solenoid driver board for any dry joins at the header pins, J3.

have you tried reseating J1 on the MPU board a few times? also checking the rear for any dry solder joins.

what voltage do you have at TP2 and TP4 on the solenoid driver board in the HV section?

seeing you have another game you tested the displays in, can you swap the soldenoid driver and mpu board? What game did you test the displays in?

sounds like you put the Playboy displays in another pin to test, what if you put that other pins' displays into Playboy?

So the display issue moves with the MPU to a different game?

PIA U11 does the digit drives. Check/compare working/non working digits at U11 PA1-PA7 during display test

Display enabled at wrong time getting junk random segment BCD? Check U20 P12 and compare to working board.

You can throw known good PIAs at it since they are socketed.

If you have a way to run a working board next to the bad one and check with a scope relative places, digit drives, segment data, and latch strobes, you can usually at least figure out where the fault is. Two signals shorted together with distinctly look different than others and etc, but you might not pick that up on a probe. Usually you can tell with a probe because it will sound different, but its not always clear when two signals are shorted together.

One of them is not going to be pulsing if your game doesn't have seven digit displays. I forget which six ports they use, but the digit should be labeled on the schematic near the connector then trace back to the pia chip.

To have one bad port be an upstream issue means a data bit isnt making it to the PIA chip. If that was the case the game would not pass the power on self test, so unlikely. There would likely be a solenoid issue at the same time.