You’re describing one of the most interesting aspects of using HR to measure intensity (and interestingly, the most common theme of these forums…)
Because HR is an indicator of how your body is responding to both stress and environment, your LTHR is a product of that environment. HR is cheap, ubiquitous, and easy to use. But, it’s a picky, picky companion. I’ve personally observed research where an athlete maintained a fixed pace for an hour with a HR of 165. A week later, the same athlete at the same fixed pace had a HR of 190. The difference? The temperature of the room.
Sleep, stress, time of day, indoor vs outdoor, humidity, when you last ate… all these impact HR.
So, when you performed your threshold test a few weeks ago which identified Pace and HR thresholds in the same test, those two where matched for that particular environment. In your new environment, HR has diverged.
My advice is to use Pace as your primary and HR as a skeptical secondary measure.
You’ve also probably improved your threshold pace since your last test, so what you think is Zone X pace is possibly now Zone 2 pace, and what you think is Zone Y HR is probably Zone 3 HR.
Here’s the hard part: clearly, there is an upper limit to HR that should not be ignored. If you are running along at Zone 2 pace, but your HR is 190, slow down. The challenge is, that upper limit is completely different for each athlete and for each zone.
When in doubt, slow down. It’s perfectly acceptable to run the RF runs in Zone 1 if your HR feels too high.