If "Reset Keyboard Dictionary" does not reset some words, what supplies them?

There are some previously unanswered questions about "Reset Keyboard Dictionary" (in Settings) not resetting the autocorrect for specific words. (In my case [iOS 11], the letter "i" converts to "A" and a unicode character... and by the way, a manual shortcut override appears to be the only fix.)

So if the issue is not caused by the Keyboard Dictionary, what other plugins could be providing this erroneous autocorrection? Do other apps contribute words to the dictionary? And is there any other way to remove them from the dictionary without wiping the whole phone?

iOS comes with a built-in dictionary/replacements which can't be modified (without jailbreaking). This is why the workaround is to add a shortcut, because it's up to Apple to release a software update to resolve the issue.