With getting fitter you need to go harder than last time. It doesn't mean your HR will go down though. It may, it may not, there are lots of factors affecting it. Make sure your tracker does track HR correctly as zone minutes depend on it. I personally, don't trust wrist HR sensors as a rule and use chest strap when exercising if HR matters.
If you exercise daily, you should be able to tell whether exercising at certain resistance or cadence feels easier. The perceived effort is one of the ways to judge whether fitness level is changing. Some treadmills have workouts and tests built-in. If yours has such just do one once in a while.
