From my local testing on Ubuntu 2004, the same behavior when I use horizon Linux client 2206, 2212 and 2303. If there has no smart card attached and the broker connection server choose "Optional" as Smart card authentication for users type, the AD login page will automatically. There is no difference on user experience on Ubuntu and is there some special settings on you local IGEL machine? Another suggestion is that, if you want user always to use smart card to login, user can choose the "Required" as Smart card authentication for users type in connection server settings.
