I think it depends on the menu (anywhere - WDW or elsewhere). If the burger description says something like special, artisanal, house, homemade, blend, signature, flavorful, custom, etc. then I probably assume something more than just a basic hamburger. If it’s a QS/fastfood type place then I’m more likely to assume a pre-made beef Patty if not otherwise noted with a description.
Here are a few specific examples, I am quoting directly from the menus:
Cosmic Ray's: Triple D burger is an Angus Beef patty"
Pecos Bill is also decribed as an Angus beef patty
Connections American classic burger is described as a "Gourmet beef blend" (had not looked at that one before posting this query)
D-Luxe Burgers serves a "Classic Cheeseburger" that has "Signature Blend patties"
City Works Burger is ""Fresh USDA prime beef patty"
Landscape of Flavors serves a dish simply listed as "Pasta with meatballs" (no specification at all)
LoF also has a Cheddar Cheeseburger that is "a signature blend Burger" Looking under the allergen menu, the burger is an option for wheat allergens, but not sesame)
Primo Piatti's cheeseburger is also described as a "signature-blend burger"
Roaring Fork serves an "Angus burger" cheeseburger
Steakhouse 71 serves a child menu cheeseburger that has no description. It is an option for wheat allergies, peanut allergies, but not soy allergies.
Contempo Cafe has an "Angus Burger"
Gasparilla Grill's offering is only described as an "artisan burger"
GF Cafe was the menu that prompted me to post his question. They serve a "Signature Burger" that is described as an "Artisanal 7-ounce Burger Patty"
Looking at all the above menus somewhat answers my original question: some locations are indeed serving cheeseburgers that are a non-specified blend.