Julian Nagelsmann's Bayern missed a chance to move 11 points clear at the top of the table, with second-placed Borussia Dortmund's match at Mainz postponed due to a Covid-19 outbreak in the hosts' squad.
But the Bavarian giants are still in pole position for a 10th straight league title.
Third-placed Leverkusen remain four points clear of fifth-placed Freiburg in the race for a top-four finish and Champions League qualification.
It appeared as though it would be business as usual for Bayern when centre-back Niklas Suele gave them an 18th-minute lead with his first Bundesliga goal of the season.
Leverkusen were gifted an equaliser nine minutes before half-time, though, in strange circumstances.
Kerem Demirbay's free-kick from the right wing was sailing straight to Bayern goalkeeper Sven Ulreich but Mueller, under no pressure, stretched full length to volley the ball into his own net.