Have you installed iOS 11.1.2
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A work around
The bug causes the letter "I" to be substituted by an invisible character, called Variation Selector 16. VS-16 is a code typically used to combine two characters together into an image, but in this instance, the iPhone does not appear to render an image, instead showing the capital A and ⍰, a question mark in a box. Others have seen it replacing the symbol with "! ⍰".
How to get your 'me' back?
To fix the "I" issue, you have to set up a manual text replacement until Apple performs a full update.
Go to settings > general > keyboard > text replacement
For phrase, type an upper-case 'I'. For shortcut, type a lower-case 'I'.
Question: iPhone 6 I is replaced with A and a question mark symbol. How do I fix this?