I just never heard of any cat taking 3 days for the reaction to show.
I've never heard of that, either, but . . . that doesn't mean it's not possible. Pookie, like your Mazy, reacted to foods almost immediately to within an hour. I don't recall him ever having poop issues as a result of food, other than when he and his sister were on the "prescription" IBD food, which generated a LOT of poop.
I'm wondering 2 things, and bear with me because this is just me "thinking out loud" -- perhaps she needs digestive enzymes (or more than what she currently gets -- I don't remember if she gets them or if it's just Mazy) to help with beef, and/or perhaps adding some ACV (again, or more than she currently gets) with the beef would help her with the digestion. Another thought, add (more) probiotics when she gets beef. Basically, I'm wondering if the beef is somehow placing more of a demand on her digestive system than the poultry, and therefore she needs more "help," whether it be probiotics, digestive enzymes, ACV or a combination of any of them. If I had to start somewhere, I'd start with the ACV, because I have a sneaky hunch that beef needs a little more acid to help break it down. I have no proof of that, it's just me having a hunch. Because, and Heaven knows, I could be totally wrong, to me an intolerance generates vomiting or regurgitating, which is the body's way of getting it out ASAP, and not so much a poop issue. Again, though, I could be wrong. To me, a poop issue means she needs some assistance with the digestive process.
All that said, you need to do whatever makes you most comfortable, and if you'd prefer to go the "elimination" route, by all means do it. It certainly couldn't hurt to try it. Another option: eliminate the beef for a week, and if the poops are normal, slowly add it back with some ACV and see if that makes a difference. Personally, I'd hate to see her lose a different protein if all she needs is some additional digestive support, so testing the beef with something like ACV might be worth a go. You can always stop giving the beef if nothing else helps.