I have this problem all the time. Casting arc lightning again or activating something (F key, chop a tree or mine) stops it. It’s caused when the spell is targeting something that is dead and not in front of you (shoot a rabbit or frog with lightning strike and keep running, the arc lightning will proc and not stop if it’s on autocast). If you continue facing the creature until arc lightning reaches the end of its visual it will usually stop. It’s an easy bug to replicate but completely harmless and sometimes funny.