Hmm, my Strava fitness chart looks the same as CTL in TrainingPeaks and WKO.
This support page:
states it uses power when available, and heart rate if not, and since I have both, I didn’t read what it does w/o power&HR.
Are you thinking of something else? Because Strava fitness mirrors TP/WKO, unlike TrainerRoad which doesn’t use the same algorithm for 6 week load.