calorie accuracy

One of the constant frustrations with the Fenix is the tendency to import incorrect HR zones from Garmin Connect whenever there is a firmware update. The last firmware update was October, which would fit with your timeline.

I would check carefully in GC your resting and maximum heart rates, your HR zones and zone calculation method, as these can affect the calorie calculation.

I am assuming the heart rate data itself (averages and normal range) during your rides is largely unchanged. Cooler weather and unwashed HR straps can lead to artificially high HR rates, especially at the start of a ride.

One of the constant frustrations with the Fenix is the tendency to import incorrect HR zones from Garmin Connect whenever there is a firmware update.

I am puzzled. Could you clarify?

Do you on purpose keep incorrect zones in Garmin Connect which the watch then imports when it gets a firmware update? If so, what is your reason for having incorrect zones in GC?

Or do your previously correct zones in Garmin Connect automatically change to something incorrect when the watch gets a firmware update?

No, I don't keep incorrect zones in GC. It does however import previously correct zones incorrectly. And the number of ways it manages to do this continues to amaze me.

I have variously seen it over a series of firmware updates reduce my maxHR to just 1 bpm over my RHR, change the method of zone calculation from %HRR to %maxHR, erase my LTHR and also just plain overwrite custom percentages.

Ok. What confused me was that you describe the problem as if it is the import which is wrong. But I guess your real problem is that the zones get changed in GC.