Here is how FF describes a fallback rise: Fallback rise: Temperatures may show a "fallback rise" pattern. In a fallback rise pattern the temperature rises significantly and abruptly as you would expect, but then it drops again immediately before it rises again and is sustained throughout the luteal phase.
Of course I have no information to be sure, but our temps could be influenced by so many things.. maybe your room was colder that day, etc.. I mean if it was just half of a tenth a degree higher you'd get CH for CD20.. just try it - go back to CD 22 and enter 97.94 or 97.95 and I think it would move your CH to CD 20 I think. I think there's a possibility you did O earlier so don't feel so down! Oing on CD20 matches better with both your OPK and your CM pattern