Have you done a Anaerobic Threshold test. You might find that your AeT and AnT are within 10% of one another. If you felt it took concerted effort in this test to maintain a HR in the upper 140s even though your breathing still felt pretty relaxed that would indicate that muscular endurance was more the issue. This is what happens when the AeT has gone up significantly. The top of Z2 gets close to Z3 so Z2 workouts can start to feel like Z3 workouts and have a similar training/fatigue effect.
If that’s the case when add more Z3 and 4 workouts. Drop Z2 and do almost all aerobic base training in Z1.
I hope this helps.