The manager of the restaurant where Oakland A's catcher Bruce Maxwell claims he was refused service by a pro-Trump waiter claims the MLB player is lying through his teeth.

Maxwell -- the only MLB player who took a knee during the national anthem this season -- told TMZ Sports he was recognized by a waiter at an Alabama restaurant after the protest ... and the guy refused to serve him.

Maxwell said the server told him he supported Trump, disagreed with the protest, and refused to serve him lunch.

But Anne Whalen -- the manager at Keegan's restaurant in Huntsville, Alabama -- tells TMZ Sports ... the story just ain't true.