I upgraded my IPhone to 11.0.3, and then it started changing "I" to a weird ? in a box. According to Apple Support, changing the the keyboard setting will fix this bug.

Step 1.  Go to Settings

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Step 2. Select Keyboard

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Step 3.  Select Text Replacement

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Step 4.  Click on the + 

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Step 5.  Type an uppercase I in Phrase. Type a lowercase i in shortcut & Save!

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So far, this solution has worked for me. But, I'm going to bookmark Apple Support just in case!