I’m really glad you posted this for us to discuss.
First, congratulations on your new threshold pace, I expect another 15 second drop in the next test.
LTHR remains remarkably unchanged regardless of fitness. Think of it as engine temperature. At 100kph the engine temperature won’t be much different than at 130kph (for similar weather conditions). Since your HR is just an indicator of how the body is responding, the relative level of stress at threshold pace remains the same even if the output (pace or power) changes.
Or, think of if this way. An olympic runner has a threshold pace of 3:00 per k. That’s 3x faster than your pace, but will their LTHR be 3x higher? No, their LTHR is going to be close to yours (and mine) between 155 and 180 (as is almost everyone’s).
Yes, your reading is a great sign. An increased outcome for a give (or even lower) HR is an unequivocal sign of an increase in fitness.
You won’t see a badge for lower HR. TP, like Strava or Zwift, uses game theory to motivate athletes to stay engaged. Higher is always better, harder is always better, which is why so many athletes stay mired in the moderate intensity rut.