That sounds a bit odd. Do you think the interupting sear is original? Just wondering if it might be a replacement & although free to pivot is there a possibility that it's top profile is such that when the barrel closes on it the force it too much over the fulcrum pinching the 'sear downwards rather than tipping it. If it is original could the sears upper surface be worn or perhaps have been re profiled? Maybe swap the one from the other rifle & try it......