The Drengin are getting access to the invulnerability fields too early. They haven't researched the preceding techs, nor has anyone else. In fact, they're matching these monstrous arrays of shields with Pulse Cannons! The result is that my frigates can't even scratch their heavy fighters despite their mass driver attacks. I don't know how they're getting the fields, but it's definitly a bug.
Also... has the off-type defense issue been fixed? I'm surprised that my frigates, with 10 mass driver attack, can't even scratch heavy fighters with 36 shields. You'd think that the shields would be overpowered fairly easily with that weapon type.