It seems like this is a bug or a breaking change:
Loops modes are:
console.assert(BABYLON.Animation.ANIMATIONLOOPMODE_RELATIVE === 0);
console.assert(BABYLON.Animation.ANIMATIONLOOPMODE_CYCLE === 1);
console.assert(BABYLON.Animation.ANIMATIONLOOPMODE_CONSTANT === 2);
Whereas this commit sets
//From
animation.loopMode === 1
//To
animation.loopMode !== Animation.ANIMATIONLOOPMODE_CONSTANT // 2!!