It's a debuff to the target that does as suggests: it cripples shields and causes some additional shots to go through. It says nothing about the effects it has on other weapon types.
Actually in the description it specifically says that it increases phase missile's chances of bypassing shields.
Not to mention the behavior is very unexpected being how that entity modifier usually works. I believe normaly acts as a multiplicative increase to bypass chance.
Which means with 30% shield bypass chance on phase missiles it only provides 6% extra bypass chance(a 20% multiplicative increase).
By contrast it's adding 20% bypass chance to weapon with 0% bypass chance.
It's definitely a bug. I could see a few arguments for why it should be left as is- and that is a whole new discussion topic which I don't think it clear cut one way or the other. But I severely doubt that the ability is working as intended.
That said though as it is now, Shield Disruption makes Designate Target(a very potent skill) look like a joke. for non-PM weapon types 20% shield bypass and -8% mitigation is a .43/.35*(1+.2 *.43^-1)= an 80% damage increase, 38% of which goes directly to hull.
That's twice the damage boost of designate target, on a shorter cooldown, with a longer duration, with direct-to-hull-damage, and to top is all off is available earlier in the game. And that's with a measly 65% shield mitigation. With 70% shield mitigations that increases to a 93% damage increase and against 75% that increases further to a 112% damage increase.
Of course this bug has presumably been in since the inception of the game and been low impact until now. The real test will be what can be done with it now that people may know about the bug and build fleets around it. I could very well see it ending up too powerful when combined with Advent Fleets or Vasari Loyals fleets built around their devastating Wave weapon upgrades late game.