When the Skeleton enters Attack Mode, the arm will turn into the Attack Position, however, the skeleton seems to behave strangely - Instead of him face the player wherever he is, only the arms moves, it causes the skeleton to look extremely strange and weird.
As mentioned, to you see this bug you have to:

• Trigger the Skeleton's Attack Mode;
• Go behind him, on his side or whatever;
• Then you'll see that only his arms face the player.