This has happened to me a few times too, I have to complete a word 3 times before it actually marks it as correct. I almost wondered if it was intentional because it happened so often.
If you delete and retype the word it is technically the same attempt and you are not being fast enough, so it is in deed a feature to practice the words you have made a mistake in in the first place.
But I can see how that can be less than obvious.
I fixed that now.