Ok well that rules out a few thoughts. Caffeine (on an empty stomach), stress, rushing etc, can drive the HR higher than expected, or needed for output.
It is possible that you don’t tolerate fasted training well, and that breast feeding and hormones could definitely have an increased effect on that. Especially the physiologic demands of breastfeeding. Unfortunately there’s no hard science to show that of course because it falls under the category of women’s health. However, your body does have increased fuel/energy demands to support breast feeding as you are all too familiar with.
The reason I asked about goals, which sound fabulous by the way! Was how important fasted training would be for you, since you do not have ADS. Having done numerous hut trips in Europe I can guarantee you will be fed some sort of breakfast every morning at the huts, can carry plenty of food since packs are light for those trips, you can get snacks and sandwiches or buy candy bars if needed and as you know dinner is served to you (oh so fun!!). You are not going to be in a position for this trip that being that metabolically efficient to push for extended time with limited caloric resources is an issue so it’s not a huge deal.
Here’s what you could try if interested. I am assuming this was done with a chest strap, forgot to ask that as well. Wrist data as we have expressed is not accurate.
If you’d want to try again –
1) wash your chest strap and see if that makes a difference, also change the battery. If chest strap is not new or you’ve used it a lot, that can give bad data readings.
2) Soft fasted, which is four hours after you last ate. Up at 6 with breakfast at 7 try training at 11am with no additional food. That’s still considered fasted to some degree, see if you get a different result.
3) don’t worry about the fasted training until you’re a good 4-5months out from tapering off breastfeeding and see what happens.
Based on everything I’m guessing it has something to do with breast feeding and go with #3, but rule everything else out above and then maybe you do just need to adapt to it and then that would be based on bigger picture goal of really needing to be able to manage big efforts with little food/fuel. It’s not always necessary ( :
I hope this helps!