Hi Ryan,
I used Garmin devices and their Connect-service for a long time and think I have a rudimentary understanding how it all works.
Most of Garmin’s health and fitness metrics are calculated on-device instead of in-the-cloud. This means you get more data by buying into the higher-tier devices or upgrading your older gear (always check which features are supported, if a certain metric is important to you).
In this case; buying a Fenix 5 “unlocked” your FTP estimate ( → Source).
It’s tricky to say which specific activity made this estimate possible, since Garmin uses (their now owned) Firstbeat Analytics and a lot of data points in your previous activities to come up with their numbers ( → Source).
I think your watch and Garmin Connect synchronized all activities including SUF (and possibly the last missing metric for the estimate) either on the day of the mountain ride or even a day after. Hence the delayed reporting.
Personally, I would trust the numbers from your Full Frontal Fitness Test and ignore Garmin’s guesses.
If your workouts are feeling too easy, maybe it’s time for a re-test on The Sufferfest.
Just ask yourself: Are-you-training-with-accurate-4dp-targets?
Anyway, just my two cents. I hope this helps.