If a player scopes in while leaning and holds the lean for a long time the character will suddenly encounter 100% fatigue, almost as if the player has been holding breath (the game function) the whole lean
an example is here, Break is leaning without being out of breath then suddenly his character jolts back with a flash of colour change on the UI and has to wait for his breath to reset
its almost as if leaning is considered a period of sprinting, or it accidentally resets the fatigue count to maximum