Zero Anaerobic Effect After Hard Run

How does Garmin calculate Anaerobic effect exactly? I have been using my Fenix 5 for a good few months now, so it?s dialling in to my training nicely now. It?s got to the point that I am starting to really rely on the data it provides me. I have also started to use the HRM run too and that is starting to give me more good data. Yesterday, however, I did a hard run in atrocious weather where I was in my maximum HR zone for a good 3 Miles. When I finished my run I got an Anaerobic effort of zero.

Does anybody know what this may be the case. As I said my HR was in max for a good 20 mins.
i did lose GPS for a short period which gave me bad data but I wouldn?t have thought Anaerobic data would require GPS data to calculate.