Without giving too much away, one of the words used as an answer to an inward clue isn’t spelled in a legal way according to the Oxford Dictionary.
All of the letters in the word (minus the letter removed to form the final answer) are correct except for one. Changing that one letter to fit a ring clue answer turns the inward clue answer into a word that doesn’t exist.
Either that, or I got extremely lucky and was able to find a completely different set of inward words that still satisfies the ring answers, because I was able to get the correct final answer.