A family of Microsoft word processing software products for creating web, email, and print documents.
I think I figured out what was going on. If you have a space to the right of the cursor and you type "i", it assumes it's a standalone word. So the problem occurs when I accidentally end up inserting text before a space. But the space may not be visible, of course. It's not really a bug: how is poor Word supposed to know that you don't mean "i" to be a word if there is a space both before and after. (I guess one solution would be to de-capitalize immediately as soon as you add some other letters.)
Deleting "i" from the autocorrect list solves this completely, of course.