Is Normal Zak no longer dropping things?

I found it happening on all bosses as of the Star Force update. At times, the bosses will drop nothing at all.

I've heard people complaining of it happening before this patch, as well.

I believe it's a form of party glitch. The game thinks you're not in the party that killed the boss, so you can't see its drops.
If that is the case, the drops should become visible after about 2:45 minutes. Try waiting it out?