Chipquick SMD291 No-clean Paste Flux - Conductive?
While flux may be pristine before it's used, its job is to clean the surfaces of the metals the solder is supposed to join, physically and electrically. So where does all the junk the flux dissolves go? Into the flux, of course, and if there's enough of it and it's the right kind of junk, it could well provide a conductive path through the cooled flux which will be removed when the board is cleaned.