No replacement has yet been announced.
MMA Weekly reported the lineup change on Tuesday.
Undefeated in his professional career, Munoz defeated Chuck Grigsby via first-round TKO in his WEC debut at WEC 34 in June.
Steinbeiss, a WEC newcomer and BodogFight veteran, has won three-straight contests over Antony Rea, Wayne Andrews, and Francisco Alcantara.
WEC 36, which takes place on September 10th in Hollywood, Florida, features WEC featherweight champion Urijah Faber taking on challenger Mike Brown and the highly-anticipated rematch of WEC middleweight champion Paulp Filho and Chael Sonnen.