Thanks all for the suggestions. I ran through a calibration cycle (complete discharge followed by full charge) and charging behavior is normal again. The issue seems to be transitory.  
I've noticed that the charging behavior on this laptop has changed. Previously when operated on A/C it would charge to the specified level (*) and then continue to operate from A/C, neither charging nor discharging the battery.  Now it cycles between charge/discharge as often as every second or two, even when just sitting and there is no activity. This seems wrong and I am concerned that it will have an adverse effect on battery longevity. Is there some place to open an issue regarding this? (*) I frequently operate on A/C so I set the charging level in the bios or using the Linux utility /opt/dell/dcc/cctk to 50-65%. After more than three years battery health is reported at 95% where it was when the battery was new.  
XPS 13 9370, BIOS 1.19.0 charging woes