I have tried everything to get it, like waiting to starve so that I start to take damage and then eat it and then proceeding to eat rotten flesh until the hunger effect goes away. I have recently just made a new Microsoft account and now ticking off all of the achievements and this is the only one I have had a problem with. I am guessing this is some type of bug or something because I have unlocked the achievement before on another MC account. I thought maybe it was something to do with the realm I was on but tried the same thing again in a private world and again nothing happened.