Remember the "B" stands for broadcast so doesn't indicate what's going on at the other end for the controller - there might have been an outage of some sort or perhaps the other traffic had no ADS-B and you were being procedurally separated.
There was a mercy flight today into Sydney this afternoon from the north west that was nil ADS-B and up at F410 from memory. Any chance it was them?
If you're told "standby" it usually means the controller has to consult with another sector or two because your level has already been coordinated. We don't always get an immediate answer when things are busy.