In my opinion - Fang He didn't want to have anything to do with...

In my opinion - Fang He didn't want to have anything to do with BCS. She only submitted the form after BCS threatened her. They even did an ASX release on 1 December:

"BrisConnections ... said today that Melbourne-based investor Fang He had disposed of her entire security holding in the trusts.

The trusts have since advised Mrs Fang He of her obligation to lodge a notice that she has ceased to become a substantial security holder in BrisConnections."

So that might explain the delay between her sale and the ASX sell notice. But it doesn't explain the lack of a corresponding notice telling us who brought them. As you say, BCS have access to the share register so they must have at least a name and address. It is a mystery.

I guess that BCS have tried to contact whoever is listed as the new owner, and either failed to find them or failed to get them to cooperate. I do wonder if Fang He did something like make up a name and transfer her shares to someone who doesn't exist. If she did then that missing 8% of shares that will not vote might make Mr Bolton's job of getting 75% of votes very difficult.