Re-reading this thread, could it be that the problem you’re having is that Siri Shortcuts won’t let you log a BMI entry? While the thing I posted earlier about height needing to be in meters is true, I haven’t yet found a way to programmatically log BMI into Health.app.
What I do is calculate BMI and copy it to the Clipboard. I then have Shortcuts open Health.app so I can enter the value into the BMI section. Interestingly, once I tap the “Add” button in the BMI section, the value copied to the clipboard is already there, so all I have to do is confirm the value.
Its not an ideal situation, but I can’t figure out how to get Health.app to allow shortcuts to log a BMI measurement.
Sorry for the mix-up.