If you are spending the vast majority of your training time in Z1-2 then your AnT will drop during this period of aerobic base building. You are not stimulating the higher intensity metabolic pathway if you are exclusively trying to cure aerobic deficiency. This is why we call this period the BASE phase.
The AnT will respond much faster to the Z3 workouts you are beginning to do now that you AeT has done to improving AeT. The 2 systems while inter-related still respond to different stimuli. Recall from the books that improvements to the anaerobic metabolic turn over are largely enzymatic so happen fairly quickly.
Are you sure you conducted the AnT test on a day when you were well rested? If you’ve begun to do more Z3 training it is possible that you might be carrying some fatigue in your legs and that will greatly affect the AnT test. The AeT test is less susceptible to specific leg fatigue because the intensity is lower.
you should still be able to use the drift test reliably despite the smaller spread between thresholds.