lvl 13 Remorseless - Increase Sylvanas’s Basic Attack range by 1. Sylvanas’s Basic Attacks against enemy Heroes with 3 stacks of Banshee’s Curse shoot an untalented shot of Withering Fire. This shot of Withering Fire applies Banshee’s Curse.
Curently it works as a bounce attack meaning it doesn’t do a shot of Withering Fire in a 1v1 scenario. So either the tooltip is incorrect or this is indeed a bug.