I agree that it was probably the beef fat. I know from messing with rendering tallow and lard that the flavor and renderability of beef fat can vary widely, and pork fat in general tends to render way faster and easier.
The best batch of burger I've made myself was 90/10 lean game meat/pork fat trimmings purchased from the local grocery store, ran through the grinder 1x with the fine plate on. Just sticky enough to make perfectly pressed burgers, great flavor, slightly chunky but palatable texture for burger dishes in the cast iron.
Worst batch was 75/25 lean game meat/pork fat trimmings from a buddies self butchered 4H pig, ran through the grinder traditional method of 2x, course then fine. WAY too much fat, sticks to everything and annoyingly difficult to press burgers out, extremely greasy and has to be drained, ground into a less than favorable texture of almost mush when cooked for burritos or similar dishes.
Going forward I may try to source some quality wagyu beef fat, otherwise it will be 10% pork trimmings.
If you're into canning, that might be a way to salvage that burger. Hot packed in some broth usually turns out pretty good and may dilute the fat content enough to not coat the mouth (I despise that texture..), or make a big batch of meat sauce or chili. Best of luck.