@Udoma I've seen the Interceptor come in before the ult and still not get bonus stacks. Sometimes he's at 20, or maybe 30 stacks, and therefore can't use his insta-kill. I always bring him in as the second reinforcement. Maybe it only happens if you've already sabotaged the hanger? I don't know, but it's not consistent. Sometimes it works properly and you start with 50 stacks.
I wonder if the cleanse from Dagger might be interfering. Sometimes I bring in Interceptor, and due to the bonus Ferocity debuffs, Dagger gets an immediate turn before Inteceptor and cleanses all of the debuffs. Maybe that is causing an issue with the number of stacks Interceptor gets.