It is unusual for LTHR to drop like that. Typically, LTHR is static in an individual, having more to do with genetics than fitness. What changes is the output for a given LTHR. Your threshold pace may be 7:00 at a given LTHR for Week 1, and 6:30 for the same LTHR for Week 9.
Additionally, HR is notoriously picky. If your LTHR for test #1 was performed outside in the afternoon a 75 degrees, but test #3 was performed inside in the morning at 70 degrees, those are not the same test. Temperature, time of day and indoor/outdoor environments all change HR for a given output (as well as when you last ate, sleep, stress, humidity, and more!). You can only compare LTHR under similar conditions.
These are two of many reasons why we recommend Pace and Power over HR to measure intensity.
Regardless, it really does not matter is LTHR is changing, dropping, or increasing. What matters is: are you getting faster in those tests, independent of LTHR?
Can you share with me the data from those 3 tests? I’ll take a look and analyze further.