It is entirely possible the researcher found something but didn't realize how deep the problem went. Apple may have released an incremental patch and is working on fixing a larger issue they found when digging into it.
When this has happened in the past, from the researchers perspective things seem quiet/delayed because we obviously can't share details of a larger vulnerability with them. All we can really do is ask for more time. In the end it all works out and they get paid out/credited for the original+follow on bug.