I have seen this on both corvettes and tactical cruisers. In one case, a T2 Aion (Orcus) completely negated the damage from my plasma ram and didn't take a scratch from it. Other times Corvettes have been able to escape from an artillery shot due to this short period of invincibility. This is completely reproducible, and appears to primarily effect single hit damage. The invincibility is no longer in effect after around 3-4 hits, or 2-3 seconds. (Estimation there, as it is really difficult to test on the Murometz due to fire rate, and the Vindicta due to the amount of guns firing which count as individual hits) Not sure what could be causing this, but other players have noticed it as well, so it's not just me.