UserID is random everytime instead of the one provided with AuthValues

We're using the following logic:

GameClientInstance.NickName = accountData.Nickname;
        GameClientInstance.UserId = accountData.AccountID;
        GameClientInstance.AuthValues = new AuthenticationValues(accountData.AccountID) {
            AuthType = CustomAuthenticationType.Custom,
        };
        GameClientInstance.ConnectToRegionMaster("EU");

Indented Result:
UserId after connecting to Photon is always the provided one (accountData.AccountID)

Actual Result:
UserId provided by Photon server is always a random unique one

Any help would be greatly appreciated