The bottom line is, unfortunately, that when you start mixing rabbits they quickly become a generic "rabbit" shape. There is truly no telling what breeds are in the mix unless you have an accurate pedigree depicting the mixing.
I've got french lop/ English Lop / NZ / TAMUK /American mixes that have huge ears that you would assume came from Flemish Giant but they don't have that breed in them.
Once it gets muddy there's no telling.
The magpie coat could have come from any rabbit breed that has that gene. Rex, mini rex, satin, harlequin, Rhinelander, all the different lops, the angoras, jersey woolies, etc. There's a bunch. (Any that have tricolor in their SOP.)
However it's not found in Flemish Giants, New Zealands, or Californians. I guessed Rex because that's more commonly a breed mixed into other meat breeds that does carry it, though it could be any of the others from who knows how far back. Also Texas A&M Composites come in mapgie and are fairly common meat rabbits in my area.
Kinda a toss up at this point. Basically all you know is "not pure flemish giant" and.... That's it.
Can you contact their breeder again and needle them for any additional information? I've seen people list rabbits as "New Zealand rabbits" but in a color they don't come in. When I inquire about it they go, "Well, there's FG in there too."
Worth asking.