https://www.theverge.com/2017/11/9/16629434/apple-ios-11-update-autocorrect-i-problem-fix-iphone

iOS 11.1.1 is here, and it comes with a small but important update: a fix for the strange autocorrect bug in iOS 11.1 that has been automatically changing the letter “I” to “A [?]” for some users.

Apple had previously recommended using Text Replacement as a manual workaround so that users could type the letter “I” again, but it seems things are finally back to normal again with the new update.