MiBand 4 heart rate too low

Hello, I am using the MiBand 4 with the Notify app (no Mi Fit).
I've noticed an issue where the heart rate is simply too low on some exercises, for example this was supposed to be a 3,5 km run and it's unfathomable to be so low, even for a professional athlete.

Whereas, I was shooting some hoops and turned my exercise to "freestyle" and the heart-rate seemed more true.

I tried cleaning the sensor this night and I'll go for another run tomorrow to test, but if you guys have any ideas, shoot. I need the data to be somewhat accurate for a project (beep test).

And also, if you know a way to get heart rate instantly when starting a workout instead of seeing a "- -" for ~30 sec, let me know, I'd appreciate it.