Apparently, there has been somewhat of an unusual discovery that Alcremie has a hidden form seemingly considered its "Rainbow Form". But trying to evolve Milcery into that form just seems very hard to come by. Accordingly, the Pokédex has data on this mysterious form as shown below.
Sword: The moment it evolved, it took on a complex flavor. This is because of the way its cells spontaneously shifted during evolution.
Shield: The cells that compose its cream fluctuated suddenly during evolution, giving the cream a complex flavor.
So, indeed, it is saying that it produces somewhat of a "complex" flavor. So this could take some time for even the best researchers to find the form.