Well... That's what happens when you feed a conservative cat overpriced food from a liberal... ;-)
We have experienced food-based issues with certain brands also. Had one cat that got bloody stools everytime we fed her whatever brand it was (Mrs. NoC knows - I just know there's a brand I'm never supposed to buy again!)
Also hasten to add glad LPOD is OK. And our cats seem to tolerate Fancy Feast better than most other wet foods. It’s just that each cat has a different favorite flavor... ;-