In 2.5 if you send a Saracen out armed only with Shiva cannon, it will fly right up to the maximum range of the UFO's weapons, then turn and fly in front of the UFO until the enemy shoots the Saracen down. I haven't tried yet with 2.6, but it's probably the same. In 2.3 (or was it 2.4... been a while since I played those versions) the problem did not exist. I'm no coder so I couldn't say what happened, but from a game player's perspective it seems the "avoiding enemy fire" subroutine takes precedence over "close in and attack." Stilettos have no such problem - load them up with three Shiva, twist their tails and send them on their way, they'll home in and take down UFOs.

I have only found the problem when using double Shiva on Saracen, against a UFO that has weapons. Put two rocket launchers on a Saracen, or one rocket, one Shiva, they work fine... or as well as planes with rocket launchers can be said to work, which is not well at all. Send a Saracen out after a scout UFO and it closes and takes the UFO down, because there is no enemy weapon range for it to avoid.