Mine did it too after the latest upgrade to that and the instinct3 (my other watch). My recollection is that I changed my primary watch setting and BOOM the body battery crashed just lthe most recent graph above.
This also happened to me and now my watch and Garmin connect app are out of sync. My watch said my body battery was 60 this morning but my app says 90. The numbers don't match up no matter how many times I restart the devices or sync through the Garmin connect app
