i've seen my own JKA with at least 2 expose on himself when his aoe didn't crit enough. It didn't used to be the case, and then it became the case. I don't know for sure if currently is still the case.
i don't have Jyn zeta'd but i've seen some videos where under her lead, rebels do stack exposes on the opponent.
but to be honest, i'm not sure what's not WAI. is it stacked expose? or continuous expose? now that i think about it, it seems more about how a Resistance skill that exposes simply cannot trigger expose and then apply it again. what a curious restriction... that is like saying if you hit a toon with a DOT with a skill that may also proc DOT, then it cannot proc again....
why only treat 3 of 4 Resistance exposes this way?