It looks to me like calls to the Admin/GetUserAccountInfo doesn't work if all I give them is a TitleDisplayName. It used to work.
Using the playfab ID, I get the info
Using the TitleDisplayName (from Console > Title > Player > Overview > Display name) it does NOT work.
I can verify that the DisplayName I get back from the FIRST call does not work in the Second call. The body I use is: { "TitleDisplayName": "Some Valid Display Name" }
Documentation here: https://learn.microsoft.com/en-us/rest/api/playfab/admin/player-data-management/get-user-publisher-internal-data?view=playfab-rest