You may have indeed been still nasal breathing at AnT. I’ve had this myself, especially with a long, slow build up. I can never do it if I approach AnT quickly, but when gradually ramped up over a long period of time, I think it’s possible to still nose breathe above AeT. I’ve also confirmed this with lactate samples, getting samples around 4 mM (a typical proxy for AnT) while still breathing through my nose.
With respect to temperature, I’ve read that the extra stress from cold can raise heart rates, but I’ve also found that, once warm internally, colder air will keep my heart rate lower for a given output. It seems like a trade off between getting the body warm enough, and then above that, keeping it cool enough. In winter races, I intentionally dress light to take advantage of the latter.