What is common is that the vast majority of restaurants don't bother to remind patrons when the tip is included, and the gratuity line on the check is typically left open. I know Hyatt Hotels room service bills specifically label the tip line "additional gratuity" to remind people that service charge is already included in the price, but that's the only example I can think of. I've almost never had a restaurant mention anything when a service charge is included.

What's not common in my experience is including service charge when the stated minimum number of people is not met. I would have spoken to the manager about that.