He doesn't have it in any of the previous comics, hadn't you noticed?
It's also missing noticeably on Dec. 19, much more noticeably on the 17th, and also on the 15th.
Beyond that, Ian hides his hand or it isn't clear whether or a finger is there unless you already know.
I assumed it was like why Ambrose was missing an arm.
Maybe Bruno did it to Tito when he was younger.