Lots of factors could have contributed to this.
Start by reading this forum discussion to understand that AeT can be a moving target.
Then redo your test this way:
https://uphillathlete.com/heart-rate-drift/ This may give a more accurate measure.
How tired were you on this 4 mile run. Localized muscular fatigue will lower AeT HR?
It’s important to factor in your perception along with the HR data.
Your swimming training and ski touring history make you an unlikely candidate for ADS. So, don’t despair.