Yeah this is a known long-standing bug they haven’t fixed yet, for some reason.
But you gotta admit, this one’s really funny! XD
The explosion like the other witches could’ve been even funnier, and the blue skill cannot be activated when Sarah is defeated. It can only be activated if Billy only reaches 0HP, and not Sarah.
It needs to be fixed in the next update or so
Maybe Season 18 is the only option. PerBlue needs to add bug fixes alongside their refreshes.