In the Valentine special events, the special modifier is that the enemy has a permanant Regen effect. However Silent Kill Valentine gets a Heavy Regen (Urgent Care? Not sure if it has another name) effect close to death. However in this mode the Heavy regen also never ran out, making the fight unwinnable without having incredible DPS because she would heavy extremely fast all the way to full HP. This interaction seems unintended as at least one of the other Valentine variants got that same Heavy Regen but it actually ran out after a normal length of time. Keep in mind this bug happened in the event BEFORE the June 8th 2017 patch, so if it was fixed in that one you can safely ignore this. Edit: also this was in the middle/yellow difficulty, if that matters.