According to Garmins website (What Is the Functional Threshold Power Feature on My Garmin Device? | Garmin Customer Support), FTP Estimate outside guided testing is generated if :
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95% of the average power for a 20-minute interval within the ride must be greater than the current FTP setting. AND
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The average power for that interval of time must be greater than the previous 20-minute power record.
This would mean that estimated/auto-detected FTP can only suggest an increase in FTP and not a decrease. However, I and many others on the Garmin forum have gotten an estimated FTP that is lower that our current FTP. In my case, neither of these criteria were present when Garmin suggested my new lower FTP. Many replies say that estimated FTP is not based on the above criteria, and instead on some other Firstbeat algorithm. I have not found any source for this statement. So, can estimated FTP decrease or is that a bug? Is Garmin estimated FTP based on the above criteria or some other Firstbeat algorithm?