I haven't seen this exact problem, but I have seen weird scoring issues similar to this on Gottlieb games a couple of times. Usually have some random scoring that adds points to other score reels or multiple score reels out of turn.

Both times turned out to be score reel switches touching on one of the score reels. Not a closed switch but rather one blade of a switch touching the blade of a different switch. Like one blade of the ninth position switch was touching a blade from the zero position switch.

You wouldn't think that it would cause something like this but if two different score reel switches are touching all sorts of weirdness ensues.

You might want to check each score reel to make sure that each pair of switches are isolated from each other and make sure that little armature wire that moves the switches is in the right spot.