In Battle Legion, your active army is constantly being matched against other players when online, and match ups can occur in the background when players are engaged in an active battle.
In rare cases, when Commanders are matched up against other opponents while actively playing, the total Rating change after the match will be negative even after winning a match. This does not mean that players can lose Rating when defeating an opponent, but rather that their army lost more Rating in background battles than they gained for a win.
The opposite can also be true, sometimes the overall Rating change will be positive after a loss.
After Commanders reach the currently available last Rank Path Step, their armies will be matched up against enemies even when they are not actively playing. This can result in a passive loss or gain of Rating while Commanders are offline.