The initial study was presuming it was already had H5N1, but we recently did actually find a positive test in beef tissue. Considering how little we are testing in general, it's highly unlikely to be the first actual one. The study was looking at if the virus was alive after cooking. If infectious is still unknown

Beef tissue from a sick dairy cow has tested positive for the bird flu virus, federal officials said on Friday.

[...]

However, there was virus present in rare burgers, cooked to 120 degrees, although at greatly reduced levels

https://www.nytimes.com/2024/05/24/health/bird-flu-beef.html

During the briefing, the agency said that no virus was present in burgers cooked to 145 degrees (medium rare) or 160 degrees (well done) – but only mentioned that traces of virus were found in burgers cooked to 120 degrees when questioned by journalists.

https://www.telegraph.co.uk/global-health/science-and-disease/h5n1-bird-flu-virus-can-survive-in-rare-cooked-meat-usda/