I was reading this thread in response to the general lack of information regarding AGC and my other thread.
I did not see this one in the clubs, good catch.
Latest 2016 Mr. Pinball Price Guide lists PP at 580 machine production, and AGB at 350 (which seems really low).
The "multiball bug" exists in all AGC games that have this function. It is a design issue based on the switch matrix inputs that go into the MPU/solenoid driver board in combination with game coding. It is not exclusively a ball trough coil/switch issue, but can be made WORSE with this problem. Literally the game can lose track of balls if more than one switch is activated at the same time. Normally, this is not a big deal, but during MB it can be problematic if a ball lands on a VUK at a critical moment for example. A ball search ends up being required. It has nothing directly to do with the switches, coils, or assemblies. Another solution for the ball trough coil is to use a stronger assembly spring to increase the resistance of the coil kick without having to swap it out. Nothing is changed or damaged.
The only way to fix this would be to add a time delay in switch game code or MPU board upgrade of which neither is going to happen. The one thing you can do is reduce the problem is keeping the games highly tuned so you do not have additional switch issues, which compounds the problem exponentially.
If you want to protect your targets make sure the foam backing is solid on the targets, and use foam adhesive circles on the front side of stationary targets. A couple people make decaled ones that do really help them from cracking and preservative their lifespan.
Spare board sets are still available from a number of sources, but for how long I do not know.
Once they are gone, they are gone, just like Capcom boards will be eventually.