Hi there!
I've just given Majka's SGT-workaround a try, and I got to say it's working like a charm!
As soon as I started SGT on my watch, I got stable HR reading with no droppings at all, although neither the Locus Wear app, nor the SGT app was running in the foreground, I saw my watchface the whole time.
So I think Majka is completely right, somehow SGT "forces" the HRM sensor's SW-side in a stronger way than the locus add-on and that way the readouts are always executed, regardless of the watches' state. (During testing my display was off, until I raised my hand, but it did not matter for Locus, the measurements were always made)
Also I noticed a significantly more severe battery drain on both my watch and phone when SGT was running along with the locus add-on, so it would be really great if this workaround was not necessary

Here is my screenshot with time on the x axis:

Oh and yes, the zero values on the plot were present until I started SGT.