My bet this is due to a dmg shield which puts u in combat when u take dmg and not a bug. have started using it on my sb, and when scouts slams me they take dmg and are interrupted for the first 3-4 secs of the slam duration.
I've made sure to check for dmg shields too, but this happens even on targets with no shields, but you're right that it would've been a good explanation.
omni-procs does interrupts for arrows also...