Mirror Modifier - Deformation Wrong

Hi, i have a problem with mirror modifier. My models are two arms and a knife. When a put “apply”, the arms deforms wrong. I have an animated armature which affects the arms. I tried moving the mirror modifier to the first place (before armature) but the problems is still there. Here is a video:

https://youtu.be/lkVLmxdN-cI

Thanks!