Welds need a super clean surface to start with, if there is any oil or rust it will form a layer within the weld where the metal won’t melt together properly. Brake cleaner doesn’t really leave much of a residue, and it is an excellent degreaser. Problem is when you empty a can onto a prepped weld surrounded by tons of rust and gunk, it’s the solvent that gets trapped in there and blasted with heat and intense UV from the weld arc that can form phosgene. It can also happen when people try to torch rusted hubs after trying to use a can to break rotors and the like loose, that’s one of the reasons they moved to non-chlorinated stuff most places.