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. 

I'VE BEEN PROFILED
TMZSports.com

Whalen says there was a dispute that day ... but it was over alcohol -- someone in Maxwell's party didn't have ID and the restaurant refused to serve that person beer. 

Whalen also told Fox News, "He is outright lying. This is really upsetting as he was given full service."

Maxwell is adamant he's telling the truth and sticks by his story.