I have never noticed any bitterness in any of my garlics, even after having them "stored" in brown paper bags in my kitchen pantry for a full year. Don't forget, in the south where I live, 60-80% shade cloth is used over all greenhouses. Otherwise, every plant would be fried within a day.
Since I sell the vast majority of what I grow, I would imagine someone would have told me if the garlic they received was bitter. I have never had that happen.