Using rotamers for mutations

I was mutating a structure using the Rotamer tool on ChimeraX. I mutated a single amino acid (Tyrosine) to a Proline using the Richardson Common Atom library. However, when I ran AlphaFold on it, the predicted 'best model' structure they gave me had a different amino acid (Leucine) on the position I previously mutated the Tyrosine to the Proline. I was just wondering if I did something wrong or if AlphaFold works differently than I thought.