Obviously, none of these are scientific studies but I don't know if there are any done in the industry.
What I'm trying to point out is: Wage theft of all types happens all the time and I've known people too poor to risk losing their job who suffered under this issue.
I'm sure it doesn't happen often -at scale-, in chain restaurants with big corporate backing but in an industry where most close after their first year and the workers are frequently undocumented or transient. It happens.