Posting here per request of Garmin Product Support.
I had noticed over the last couple of weeks that my morning Body Battery value was lower than I would expect, as it was running in mid-50s. On the morning of 1/3, I rebooted the device (held power button until it shutdown, then powered on) and my Body Battery jumped from 51 to 83. The next morning (1/4), the value again seemed low, so I rebooted and again the value jumped from 46 to 72. On 1/5, the waking value seemed to be ok, rising from a bedtime low of 19 to 70 in the morning. However the next morning (1/6), it was again lower than I would have expected, only going from a bedtime value of 13 to a waking value of 43. Upon reboot, it jumped to 86. I would think that after a reboot the value could differ, but I wouldn't expect that large of a variance.
I have a Fenix 6 Pro Sapphire, currently running v24.10.