Rickochet No personal meat experience here with Costco, but it seems like this could be a one-off thing. I don't think it's wrong to contact them, and position it more like you're asking about whether the supply line changed...
"Hello, I've been buying large quantities of beef from Costco for years and always been thrilled with the quality, but I've worked through my most recent bulk purchase over the last six weeks and been disappointed. Can you tell me whether there has been a change in the primary supplier, so I have some idea as to whether I should expect this same quality of meat in the future?"
Even if all they do is answer the question, it's good information, but I think you're giving them an opportunity to rise to the occasion and offer some consolation for your poor experience. I wouldn't be surprised if they at least comp'd your membership for the year, or gave you a $100 gift card based on the quantity of meat you bought. Shit does happen that is difficult to manage sometimes, and organizations that genuinely care about the customer experience, as Costco seems to, should appreciate honest feedback and try to offer satisfaction.
Calling and demanding restitution might be a little dicky, but handling it like you're concerned about the quality of the product in general feels more like you're looking out for them as well as yourself.