iOS 11 autocorrect bug typing the letter capital I
This is described by Apple:


https://support.apple.com/en-us/HT208240


I think this must be a bug in the system for suggesting emoji in Predictive Text.  The mystery character is FE0F, Unicode Variation Selector-16, which is used to signal that the preceding character should have emoji instead of text presentation format.

Solutions / Workarounds:


turn off predictive text
use a 3rd party keyboard
Settings > General > Keyboard > Text Replacement


Set up a text replacement small i to capital I.

Apple has now issued similar advice.

Many people are waiting for an iOS update, but you could try backing up your device and erasing all content and settings and then restoring your backup. That should fix the issue for you at the cost of making sure your backup is encrypted and has health data and all apps are still available for redownload.