So I have been doing HR drifts every time I am on my home treadmill, because why not, and this exact thing happened to me awhile ago and I realized it had to do with my leg fatigue. You mentioned leg blasters. I’m not sure if your still doing them but that may be the cause of your high HR drift.
So I could do a HR drift one day and get 3% (good), but then if I did that same treadmill HR drift (with EXACT same speed, grade, and pack weight) the day after leg blasters, ME, or very arduous approach I would notice my HR drift was much higher (like 7-8%. I would just let my legs recover and do the same test 2 days later and it would be back to the same HR drift (3%) or lower.
By all means I think it’s still important to do the aerobic capacity work the day after or even the same day of the leg blasters if that’s needed to get the weekly volume. But if your looking for a number to anchor your AeT on I would make sure your legs are rested.
Beginner wisdom here so take it with a grain of salt.