The Welsh international appeared to collide with Argyle defender Mumba in the penalty area during the first half at Elland Road, but referee Josh Smith waved away Plymouth's appeals for a spot-kick.
Speaking after the match, Schumacher said he could hear the contact between the two players and was informed by the fourth official that the pair had come together innocuously.
"Another big call has gone against us at a big ground and that seems to happen every week," the 39-year-old began.
"The one that was close to me was Bali when he gets into the box, you can hear the contact, the two lads' boots collided. You can hear the contact, there was no contact on the ball from their player, so again I can't understand how it's not a foul.
"Trying to ask the referee and the fourth official at the time why isn't that a foul and the answer we got was that it was just a coming together. Now, if it's a coming together in the box when our player's in control of it and the Leeds player doesn't make contact with it, that's a foul, it's a penalty."
Schumacher's thoughts were echoed on Sky Sports' Soccer Saturday programme by former Premier League referee Mike Dean. He said Ampadu's challenge should have resulted in a spot-kick for the away side.
“We’ve seen the replay from behind the goal, he doesn’t play the ball, it’s a penalty,” Dean said.
Leeds had penalty appeals of their own during the 2-1 win at Elland Road, none of which were awarded. Daniel Farke suggested at full-time that he should feel the more aggrieved of the two managers, owing to decisions made the match official.