The heart rate on my venu 2 plus is inaccurate unless I'm in an activity, where it seems the sensor is "powered up" more and extremely accurate. Why is this? Can you please fix this Garmin

I discovered this morning that mine is doing the exact same thing.  Nearly spot on accurate to my chest rate sensor but only when in an activity.  Any ideas?

That's a documented feature. If it were in the powered up mode all the time your battery would drain even faster, as it does when in an activity.