Hi there. I'm an owner for just one week now, but this is similar what has happened with my Sony Smartband 2, which too had the Firstbeat system behind its HRV. This might be a long story of the whole week, and the picture is from the sixth. So don't look at it yet. :D
The first three days were OK. I know I am having troubles with stress and recovery, that's the main reason for this device. My starting BB was ~40, and nightly charge only 3-8 units. Then came the fourth day at 12 am. I went to sleep around 22 and had recovery. Right after the day changed, my stress went to high and BB plummeted. And there they were for the whole day, just stress rising for a bit when I was awake. I surely didn't feel that bad, so I read these forums and decided to reboot the device. It helped. BB went to about 40 again and stress dropped. The next night I recovered and BB rose to almost 50!
The fifth day looked how I felt. Then came this day (the picture). Night - OK. I woke up too early for I am having a day off, but my partner left early and I couldn't sleep more. The battery of Vivosmart shouted and I put it to charge. Then what - my BB went down and stress up. I was literally spending that time on the sofa. After the walk I went back to the sofa to lie down and almost fell asleep. Then I made lunch. And then, after trying to google wtf is going on, I decided to reboot it again. And now kinda back to normal, similar to previous days.
Now come on, what is this? I don't want to keep rebooting Vivosmart, because pairing it back to the phone is a pain in the ass, and you have to put every setting again (not to the app itself, luckily). If this continues, the next time this happens is going to be tomorrow (3 days - 2 days - one day??) and I'd say ugly things for that.
No drinking alcohol, no hard exercise (as I am recovering from/battling exhaustion), a good diet, drinking water...
So why does this keep happening with the devices? Is it me or? Is it because they don't get data of me really working out and therefore get messed up? But now why with after charging it?!