Originally Posted by
demue
The problem here is that according to Amex MR, e.g. in the US, this is actually permitted, but it goes against KF rules.
Add that certain bloggers even state explicitly that it works (like Sebastian on Loyaltylobby just the other day) and is fine / allowed which is not true any longer as OPs and several other recent experiences show.
Not so. Amex MR permits transfers to authorized users on the account. Before you can transfer to a frequent flyer programme, you have to set up the account on the Amex page by inputting the frequent flyer number and the name on that account. Amex restricts the field for the name to your authorized users only.
When you want to transfer you then select which of the frequent flyer accounts you want to transfer to, and Amex transmits the name and details of the account as set up to the airline. This is to prevent precisely the problem that has arisen in OP's case; by transmitting to SQ the name of the authorized user, Amex ensures that there is a "name match" on the SQ side. As far as SQ can see the points are coming from an account tied to the authorized user because that is the information Amex is giving them.