My game had a high score of 600000 and it was not updating after beating it. Beat it several times with 700000 and 800000 scores and it stayed the same.
So I've been suggested to reset the high score to 0 which I did. Played a few games right after that and the high score was updating fine, good, problem solved !
But here's the fun part, sometimes the problem is not necessarily what it appears to be.
A few games down the road I finally cracked the million points. But to my great surprise, the million digit did not light up.
Started the display test and NONE were working, all four displays were missing the million digit.
So you've guessed it, high score was probably a few millions over 600000, hence why it was not updating

The real problem: all four displays in the game are Bally 7 digits displays. After a little search, I have learned that Bally and Stern used a different pin for the million digit (one used pin 11 while the other used pin 12, don't remember which is which). Also Bally displays have commas after the thousands while Stern does not.
The fix is simple, you have to put a jumper between pin 11 and 12 on the display (actually solder both pins togheter on the underside).
Jumpered all four of them and I now have nice 7 digits displays working perfectly.
As for the commas, I don't mind them but you can remove them by cutting the base lead of Q22 (I won't).
That's it. Good night.