Just a suspicion which you can confirm or rule out.
Swing the bars around, and see if the brake hanger hits the top tube before the handle bar does. If so, that's your problem.
When you handle the bike (not when riding) the front wheel swings around, bumping the hanger. This moves it a few degrees, then moves it back when it bumps in the other direction. The repeated bumping and small back and forth movement arc works the locknut loose.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.