Body Battery and Stress disappear after charging

I have an Instinct Solar for around 4 weeks. Whenever I charge it via usb, the body battery and stress both disappear - that is they have no data from the time I disconnect the watch from the usb lead. They don't reappear how ever long I wait. I can still see the Body Battery and Stress from before I started charging the watch.

Resetting the watch by holding down the CTRL button and starting the watch again fixes the problem. The Body Battery and Stress data start appearing again from that point onwards.

Is this normal behaviour?  Do I need to turn the watch off and on every time I charge it?

Thanks