Yeah, I don't think it is necessarily targeting another spot. I think there is a maximum angle which barta can be fired at. I have a habit of jumping in the air and floating for days while raining techs on some larger bosses and sometimes barta will overshoot them if I'm too high.
I can't say I've experienced this with Foie but I generally am not as high in air against mobs that I use Foie against. Charged Foie on the other hand, can get stuck on objects between you and the target and will explode prematurely(sometimes hitting other targets) as a result. And zonde is well, zonde.
As far as random targeting, if my target dies and I don't realize it, i turn my character and end up launching the tech into the nothingness behind me.(I've gotten into the habit of releasing my techs while moving to the side for the side movement.) Or it targets something else randomly. Also it's a bit frustrating how trying to change the lock on target does indeed seem to flick around randomly. I'll lock onto a boss's foot, press up to get to the chest/head/weak point and suddenly be fighting one of the many attending trash mobs.