Didn't Apple acknowledge this the other day? That, or I saw a news article talking about how the 'I' turns into something else. I don't know if there is a fix for it.
I do find the idea of your phones concious actually refusing to let you type a letter very funny. Like it's trying to get your attention, or remind you who's really in charge here.
According to Gruber/Daring Fireball, this is apparently a machine learning mistake that has to do with the way Unicode encodes emoji as strings of characters. It "learned" an association it shouldn't.
I never even saw this bug because I went right to 11.2 PB as soon as 11.1 came out and somehow it never seemed to surface for me (That I encountered) during the 11.1 PB.