Never seen the left side kickout issue on mine and I can's simulate it either. Could the top long flipper not activating in game over be incase one of those top pocket switches is stuck so to prevent the motor spinning 24/7??
Maybe the ideal way is to run the long flipper motor for 15 seconds max on powerup if one of those pocket switches is sensed as closed.

Not quite, in freeplay my method allows zero credits, but when they're zero the credit display goes blank. "00" credits is never displayed. Credits are always reset to zero at the start of a game, and anytime you accumulate credits they are displayed. It adds incentive to see how many credits you can win in one game. I also want to add the maximum credits achieved to show when "high score to date" is displayed like a highest credits to date. There's just too.. many.. projects....

These are not show stoppers:

Bally Eight Ball and Black Jack, not that there's any way it can happen but if the first score on a ball is the spinner the game will tilt.

Bally Freedom, Night Rider, Evel Knievel, Eight Ball have a tilt exploit. Lift the machine so the ball in play tilts. While holding the machine up, allow the ball to go to the outhole. Provided you're still holding the machine up, the next ball will be served with tilt off allowing you to slowly maneuver the ball and score at will. Factory Eight Ball fix is on IPDB.

Bally Playboy, trivial really - in attract mode the displays cycling between player scores, blanking and showing to highest scores to date sometimes gets out of sync (noticeable in the first minute of powerup).

Yes, they've been fixed. And recently when I was looking at Sterns Stingray/Pinball it also has the switch bounce issue with the drop targets (now fixed) so you're probably right about the other MPU-100 games having the problem.

Is there a particular game you have in mind?...

My Playboy did the same thing every time about 30 seconds after power-up.
It is kind of a bug. The display attract sequence is controlled by a zero-crossing timer. The display interrupt on occasions causes the attract sequence to skip a check and it gets temporarily stuck on the previous attract display instead of moving to the next.
It can be fixed.

Yikes!
So this is how it affects game play.
Say you've set score thresholds to award an extra ball and for argument sake a threshold is set at 240,000 points.
You start a new ball with a score of 239,990 points. You plunge the ball and it goes over the very top rollover for 10 points as the first score.
Your score rolls to 240,000 but you don't get awarded your extra ball.
This is what RAM $4C is used for.

Here's another related bug but it's not due to that instruction.
Imagine the previous scenario, but you got an extra ball and are served your new ball with the shoot again lamp lit.
You plunge the ball and it goes over the very top rollover for 10 points as the first score.
Your score rolls to 240,000 but you are then awarded 60,000 points extra because the shoot again lamp was already lit at the start of the ball (and stays lit as it should) but the 60,000 points award shouldn't have happened.
I suspect this one might happen with other Ballys of that era.

I've had a slightly similar issue with Bally-35 games, happened to me for the third time just yesterday on a Speakeasy (first two times a few years ago on an Eight Ball Deluxe and a Centaur). The games lock up with all blank displays when reaching the first award score threshold setting in book-keeping when there is some value in non-volatile RAM that's corrupt. Disconnecting and reconnecting the battery fixed it on all three occasions.

It happens on factory code too by the looks of it.

What address is this at? I don't see any consecutive dex instructions in the code.

Oh, that's not code, it's data.

$09, $09, $09, $09, $09, $09, $00, $09
'99999909'

Haven't checked what the first $09 on the right is for but the remainder leaves "9999990". The previous gen -35 OS has something like this where that data is used as a reference threshold score to test against if the player has rolled their score over.
However Xenon, Skateball and Frontier which use this -40 OS do not reference that code at all... They instead jump out and handle it in the U2 game ROM.