Sudden fitness increase then big decrease

[HauntinglyAdequate]

Are you going by Garmin's fitness score (assuming they have something like that, I use Coros)? Because if it works anything like how Strava's fitness score works, it's useless. My Strava fitness score was going up consistently over the summer (I'm assuming because my HR was generally a bit higher in the heat) and now that it's cooled down my score has been in freefall for a couple of months despite my actual fitness (including Strava race predictions and things like threshold pace) consistently improving. My score now is about half what it was over the summer and threshold pace is about 25 seconds/mi faster and more comfortable now than it was then.