This is probably one of the more common issues that I've seen on these old Ballys. The way these work is each display is it's own module capible of displaying a score or whatnot. Sometimes what can happen is that segments can get stuck or not light up at all. This one is from a Kiss pinball.
Essentially, the way that these modules are driven can be split into two parts: digit driver and segment driver. If a certian digit is not lighting up or is stuck (many times 8 because of all the data from the other segment) then you probably have a bad digit drive transistor.
In this instance it is a stuck segment. We can see that the bottom segment, or segment d according to the schematics. Looking further at the schematics we can see that segment d is driven by Q16.
Replacing Q16 ended up correcting the issue. There other failure points on the module, such as cold solder joints on the connector and a bad BCD chip. Reflowing the solder joints on these modules should be a given. It is very important to do such otherwise you will get inconsistencies ranging from improper display data and flickering digits all the way to nothing at all.
When the BCD chip fails, I've seen it to where it will display either all 8's or nothing at all. But individual segments can get stuck as well potentially.