The mechanics and "Close flipper M-B relay" seems to be working correct. For the flippers to stay closed, these switches need to be adjusted correctly:
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There's two NC switches labeled "Close flipper solenoid EOS" in the schematics. Adjust the solenoid EOS so that the locking claw has enough time to get hold before power is cut, and the relay EOS so that it cuts power at the same time or later than the solenoid EOS. The "Close flipper M-B RE" switch should have the smallest switch gap possible for fast reaction time.