Was out flying over the village earlier when I got a warning of another manned aircraft in the area and could also hear it. I was at 103m so dropped down to 59m and switched to the telephoto lens to catch it passing over. Not an easy thing to do...
Unfortunately the screenshot doesn't show us voltage levels, but it does, as you say show you back at 87%.
But it also shows that at the time of the screenshot you were slowly descending and not moving very fast, so whatever sag had potentially...
1. How cold a day was it ?
2. How old are those battery packs / how many cycles on them ?
Most insights to what caused this should be available in the flight logs and on the battery status page within DJI Fly app. We need to know what the cell...