If JB weld if filled with iron, which is a ferrous metal, encapsulation conductors or electronics in it would definitely have an effect on inductive reactance. So, any RF or signals with AC components could certainly be affected.
You also have to remember a simple multimeter is using a DC current to measure resistance. So, the conductivity of metal filled epoxies like this may be quite different if we're talking about AC, RF, or pulsed signals. If the metal particles are encapsulated by the polymer, that would prevent them from conducting DC voltages below the breakdown voltage of that insulating barrier, but they'd still act as a chain of small capacitors. So, that could certainly make it conductive to non-pure DC signals. To what extent and what result would depend on many factors. So, if we're just talking about encasing the leads to an igniter, which is only going to see a relative low steady DC voltage that's probably fine, but things like flight computers and transmitters and receivers are probably not a good idea.