Autofill works by suggesting words that match what you spell. If you want "Coniferous" and auto fill suggests "Coneflower" when you type "C" then "Co" and then "Con". As soon as you type "Coni", the suggestion "Coneflower" will go away. And if any words that begin with "Coni" will be suggested. This is how Auto Fill works and is intended to work. If you like the suggestion, accept it and you are done. If not then keep typing the word that you want.
Auto Fill is very useful for Titles and Captions that often repeat for several similar images. If I had 40 images of the same subject and wanted to us a Title that I used before (like "Mission Trail Riders on 610 loop", all I need to do is type "Mis" and the whole Title that might have been used last week or last year appears and I don't need to retype all 32 characters.