I'm not totally sure on all Gottlieb ball lift machines, but on the Gottlieb ball lift machines I have, the ball release solenoid stays energized until a playfield scoring feature is scored. When the first points are scored, the ball release closes and prevents the first ball and subsequent balls from returning to the ball lift. If you can post a segment of the schematic showing the ball release circuit, then we can see how the ball release should work...

From the suggestion of johninc , if you made sure that the switches on M relay and D relay are clean and adjusted properly, you'll need to check the circuits that control the power to the M relay coil and the D relay coil. Also check the On Ball Release switch to see if it is clean and adjusted properly. Check for any stray or shorting wires on the relays the could affect the operation of the relays...

The test with the jumper wire showed that the ball release coil / solenoid was working properly... We might want to look at this a different way, meaning what circuit keeps the ball release coil energized? The schematic should show how the switches are set after the machine resets and the game balls are at the ball lift waiting to be lifted to the ball shooter lane.

Following the circuit back from the ball release coil, the first closed switch is the D relay switch. I can't see some of the wire colors on the portion of the schematic posted, but the next switches in the circuit would be the X relay switch, the T relay switch and the last switch would be the S relay switch. So, you would want to make sure switches on the D, X, T and S relays were clean and adjusted properly...