I found a workaround: specifing both length and columnDefinition the @Id works correctly, and the table is being generate correctly, for example:

@Id
@Column(length = 32, columnDefinition = "BINARY(32)")
private byte[] hash;

So my question becomes: since hibernate knows the length of the column and since it knows that it's using mysql (from the dialect), why it needs the explicit columnDefinition when it could add it himself and leave the source less tied to mysql?