It does sound like your warm-up may have been too hard. The fact that you got to 3.0 so quickly, did so at a low heart rate, and then dropped to ~1 after more activity suggests that an easier, longer warm-up may be what you need.
I don’t think you need to wait a few weeks to retest. Whenever you feel recovered, test again and warm up slowly. Then make each test stage increase in ~5 beat increments. You want to “sneak up” on your AeT without jumping right past it.
Also, I found it pretty tough to test strictlky by HR when running or hiking outside. I eventually switched to doing all of my testing on a treadmill to keep the stage loads more controlled.