I don’t have this issue - in the widget menu and when you access the HR widget I see a second or 2 where it’s “measuring” and then I see a pretty accurate value. I do have 24/7 HR enabled. I’ve not tried having an HR complication visible on a watch face but I can’t imagine it would work differently.
I did used to see this behaviour on my 5 Peak, though, because it took time to fire up the HR sensor and get a stable measurement. The 5 Peak only used to sample the HR every 10mins.