Do-not-disturb randomly turning on/auto-detection of activities is inaccurate

Vivoactive 4 is better looks wise and on a few other features than my earlier vivoactive 3. I am facing two recurring problems though: 1. Do-not-disturb gets turned on randomly and hence I stop getting messages/calls. I then have to manually turn it on, when I realize this. 

2. I have set auto-detection of activity at 10 minutes for walking. However, it recognizes activity much later - from 15 to 30 minutes, even though I am walking briskly without a pause. 

Any suggestions? Thanks :)