Mk6 getting double instakills

With the abundance of mirror matches we have to run in arena, I have notice a possible bug with the Mk6.  Seems to happen after opponent reaches ultimate.  Sometimes I'm in decent shape to pull off a possible win, then the opponent brings in Mk6.  He instakills one of my ships as expected, yet is still at 40+ stacks.  Even if I swarm attack him to knock him to 30+ stacks, by his next turn he seems to magically jump to 50 stacks again and nukes a second of my ships.  This has occurred several times now.  It doesn't seem to matter which ships have in the field, and whether I decided to nuke bomber or dagger on the opposing team.  Even when I get lucky, and AI tries to nuke my Mk6, it is still at 40+ stacks and gets a second instakill attempt on its next turn.  Is this WAI?  Have I missed something in the kits that allows it to either come in with around 90 stacks or something about how it keeps so many stacks after performing the instakill?