Brand Ult Bug on Karthus

I would ult karthus with brand and my ult would not bounce to the minions it would just stop on karthus. Then i ulted him later and it worked. Tried again it did not work. Then i realized when his defile was on it would stop my ult from bouncing but if it was not on it would bounce.

Hope this can be fixed or was a one game issue.

Thank you