What makes you think it was flagged for a large tip specifically, rather than just an unusually high transaction?
It still confused me how they would know it was a $20 steak and $80 tip versus 5x $20 steaks and no tip. It would appear the same, a $100 transaction at Bob’s Steakhouse.
While low risk, I think index investing still counts as gambling. The house maybe too.
Depreciating investments like a car should be ok though.