iOS 11 has been fraught with issues and they just keep getting worse. This is particularly true on the iPhone 6 and 6S. The most recent iteration is the autocorrect from I to A with a weird symbol. While Apple knows of the issue, they haven't fixed it yet. This helpful article from Lifehacker will walk you through putting in place a temporary solution until Apple finally fixes it. Here are the important details from the article:
To set things up you need to first go into the Settings menu on your device followed by General, then Keyboard, then Text Replacement.
From there, press the + sign and then type an upper-case “I” as the phrase and a lower-case “i” as the shortcut.
After you’re done, when you type an i your device will once again autocorrect it to I.
Apple plans to fix the problem in a future software update.