Can't update IDP link for an existing user

Hi everybody,

I'm trying to update the KC user Identity Provider Links section by adding a new link to an existing IDP (new Identity Provider Alias same Provider User ID and Provider Username) 
for a user that already owns an Identity Provider Link to another existing IDP (added before when creating the user itself).

The user trait update has been performed by a userResource.update(userRepresentation) call where userRepresentation has been changed by adding 
a new FederatedIdentityRepresentation, with something like

federatedIdentityRepresentationList.add(federatedIdentityRepresentation);
userRepresentation.setFederatedIdentities(federatedIdentityRepresentationList);

The issue is that I'm trying, that is the user aint't get updated and no error raised.

Does someone already tried to update a user's Identity Provider Links by adding a new link?

best
Massimo