From that assumption Im inferring that the 302 contains something that incriminates one or more individuals who are at least friends of the RATS. And Im further inferring (because of the delay in release of that particular 302) that some sort of unstated (legal?) action is being contemplated as a result of that 302s information AND THE FBI DOESNT WANT TO TIP ITS HAND PREMATURELY. Surprises upcoming?
I'm not sure about excluding all the ranking members, as Schiff is part of the “gang of eight,” and would normally be entitled to see it.
But it may be as you say - if so, I'd guess that it may relate to charges that Schiff has been caught leaking, as he was accused during the testimony of Don Jr. regarding the Trump Tower meeting which occurred before the committee a few weeks ago.