I'm with George. If there were that many fish stocked in the pond, there's no way the LMB would be that skinny if they were stocked with the other fish. Too many fish of the correct size for them to eat and they'd be looking like footballs.
My bet is bucket stocking. Even behind a locked gate. Unless you are at your pond 24/7 or have cameras viewing it, kids or helpful relatives will dump fish in it. I'd do some questioning of anybody that has access to your pond, and take a look at whomever lives in any houses within 1/2 mile (or more) of the pond. The less times that a pond owner is at their pond, the chances of bucket stocking gets higher.
A client, who has ponds 3/8 mile from any house, and the only kid there is about 7 years old had two 13" LMB put in one of the ponds that didn't have LMB in it. They were first seen chasing small fish near shore. They were skinny like that one, and I KNOW that they weren't in the pond before, as there were only forage fish in there the previous year.
