The biggest problem may not be whether they use beef that's been previously frozen, but how long a patty spent in a warming tray. A place I worked at had foodservice frozen burger patties in the company cafeteria and they were reasonably good. Probably not the greatest quality beef. But it didn't go on the grill until I ordered it at the counter and I saw a raw, frozen patty go on a flattop griddle. So frozen beef was fine if I got it right after cooking. But already fully cooked in one of these things for up to an hour? They get that pale, light brown color and they lose a lot of moisture that simply can't be reconstituted.
When I was a kid, McDonald's locations would generally just churn out everything continuously and put them in a styrofoam clamshell sitting under a heat lamp. The styrofoam seemed to defeat the purpose of the heat lamp though. Regular hamburgers and cheeseburgers just went in paper though, like they do today.