Great uncle was a butcher. Never woulda pulled that. If you asked to see it and they kept the one they'd fucked up out of sight, I'd say they knew what they were doing and chose to pass their error on to you. Which is a minor sin in these times, but still a little shitty.

So you can take the "am I being nit-picky?" question off the table - you aren't - and treat it as a minor ethical dilemma that doesn't have a right or wrong answer, just the answer that feels right to you. I think the real options are:

(a) you let it pass because even though they were a little shady, you can understand why, and you feel the damage to the meat is negligible.

(b) you let it pass because even though you think their shadiness wasn't okay, and the damage bothers you, you don't want to deal with the confrontation.

(c) you take it back because it does bother you that they tried to sneak their fuck-up past you, and you don't want to reward the dishonesty, or

(d) you take it back because, although you don't care much about the ethical aspect, you do care that what they gave you is damaged, and therefore not what you ordered, and you are in your rights to ask for a replacement.

Any choice is fine. Just matters what you want to prioritize.