UB,
There is certainly a disconnect between your threshold test and long run, almost certainly has to do with that humidity. For example, you averaged 184bpm for a 4:39 pace in your threshold test, but at the end of a long run you averaged 180bpm for a 6:10 pace (that’s a 2% difference in HR for a 25% difference in pace).
Cardiac drift is normal, even for the fittest athlete in perfect conditions. That level of cardiac drift is high, again, probably the humidity, but HR can be influenced by many other factors as well.
In cases where you are trying to use Pace as a primary and HR as a secondary, the “rule” is not cross more than one HR zone. So, if you are doing a Zone 2 pace run, and HR drifts into Zone X, that’s OK. But, if your Zone 2 Pace run drifts into Zone 3 HR, then slow down.
David