vepn, thanks for posting! This does not seem out of the ordinary for me for two reasons:
1. 5.75 minutes to 5.25 minutes is 9% faster. Your lower Zone 2 HR increased by 12 beats, or 7.5%. So, you got 10% faster in pace for a 7.5% increase in HR. Perfectly reasonable.
2. HR is significantly influenced by your environment, for example about 1 bpm per degree over 70 F. That means for a given pace, your LTHR will be 170bpm at 70 degrees F and for that exact same pace 180bpm at 80 degrees F. Therefore, unless you run your threshold tests under the exact same conditions (temperature, humidity, time of day, stress, sleep, proximity to food, music…) your HR and Pace will never be perfectly in sync.
That’s one reason why we recommend Pace and Power over HR in general anyway.