Does anyone know what the break in the Minimum Airspeed Restriction at Mach 1 is about?

Yes, Google for "Mcrit" or critical Mach number. It's the free stream airspeed at which any part of the airflow over the aircraft first becomes sonic. Usually the inner part of the wings near the root, the thickest section.

Airliners have the same but it happens below M1.0 because of the thicker wings. And it causes a much bigger change in drag and lift compared to an aircraft capable of super sonic flight, enough to make it impossible to take an airliner super sonic without losing longitudinal control and crashing.

That's the flight envelope chart, I went right to that page as well.