In what health scenario do you need your HR to stay at a constant 150 so stringently?
Further, the only way you can accurately measure your rate of improvement is by comparing average watts across time at a fixed HR. So if your HR isn't at your target level then not only are you not going to be making progress at an acceptable rate, but you're not even going to know that you're not on track until it's already too late.
And I certainly expect the app would easily be able to show you average watts vs heart rate over time. Given weight, height, pace via GPS, and heart rate for every run you could definitely do the necessary analytics.
And that the gobblygook of UT1, UT2, etc, is actually meaningful.