I too have been tractored a moment after hitting antimatter spread on a borg cube that previously had me tractored. I have a theory on this:
Like scramble sensors, antimatter spread causes the enemy ships to cycle through friendly and enemy targets randomly. That does not make you immune to their targeting, it just means they also target other stuff too. If they were to randomly target you after you hit the spread, they could tractor you anyway and the active tractor beam is not dependent on maintaining target lock.
I've noticed AM spread being far far less effective than it was not so long ago. It was great using it against the hordes in fleet actions and so much fun against the crystal shards
I mainly used it as a tractor breaker, which it used to do absolutely reliably but right now it doesn't seem to like breaking them. Hell it isn't even confusing enemy sensors anymore, almost every ship I hit with it (within 5KM of me obviously) used to stop and target something else occasionally their allies but now they don't even stop they just keep on firing at me.
Are you using saucer separation at the time that you try to use AM Spread? I have noticed that when I am in saucer separation mode (at least on the oddy, assume Gal-R is the same), the saucer will use AM spread. If there is a 30 second immunity and you then fire your AM Spread before the immunity expires, that might explain the lack of an effect. You may have to wait for the saucer's AM Spread immunity to wear off.
Glad I'm not the only one that noticed it is now lacking a bit. It seem that the Klingon side has all the fun functioning toys while the Fed side gets a few, but then they get nerfed.
Oh well, hopefully its something they missed and they fix it.
Uh, this is totally untrue.
Heck, basic stuff like vo'quv's bird of prey launched vehicles were broken for a while, and my Varanus' repair drones are frequently inattentive. (Heal me now... any time you feel like it... hello?)
Also, it might be my imagination, but it seems that Polarize Hull I isn't doing as well as it used to. I've popped it a few times to break out of a Cube tractor in STFs and... sit there. For a while. It seems to be reducing the time tractored rather than just snapping it.