2 AeT tests—no clear result

I’m in the first few weeks of the Intro to Ultra training plan and trying to determine my AeT heart rate–without much luck thus far. I’ve done 2 tests now at increasingly higher heart rates and each time I’ve basically failed to get almost *any* drift.

For reference, about 6 months ago, I completed a half marathon in 1:30 at around 172hr. The data below is admittedly from a wrist monitor on my watch–but it’s been pretty consistent without much variation, so I’m hesitant to ascribe any problems to that. On the lower end, I’ve also done a handful of longer runs at around 145-150, with very little drift.

Should I do yet another test? Call the top of Zone 2 at 170 and just train at closer to 155? Any suggestions would be helpful!

Here are the numbers, and some screenshots for reference:

First test:
Pa:Hr: .84%
avg Hr: 159
avg pace: 8:12

Third test
Pa:Hr: .27%
avg Hr: 170
avg pace: 7:23