Aha!
Opie's post made me think of it.
It works fine in landscape mode. I can type every letter on the top row after "donat" in landscape, but not portrait. Apparently with the extra room per letter, the autocorrect doesn't make the 'good' letters target areas overlap the 'bad' letters completely.
In landscape mode, the 'good' letters still have large target areas, but not so large as to completely obscure the 'bad' letters. (Try it, open Safari, flip to landscape, then type "donat" (in the address or search boxes works,) then press 'q', and very slowly slide your finger across the top row. You'll find some letters have very small target areas, some have large target areas. My guess is that in portrait mode, those 'bad' letters have shrunk down to *zero* target area.
Great catch!!