I've seen the meat version plenty of times too, it's not like I couldn't understand that either. That was the entire reason I checked the ingredients, which had never failed me until then. Restaurants normally list something about the meat if it's there... but not always.

Which I point out here because there are people saying "just check the ingredients" and, no, this clearly happened despite that.

And which I also point out because the same people arguing "just check the ingredients" would get mad (possibly even more so) if you actually agree and decide to require reliable ingredient lists.

P.S. The top result I see for "what is a mushroom burger" on Google is meatless: https://www.recipetineats.com/swiss-mushroom-burger/