I got this running php maintenance/migratePass1.php on two local wikis configured for "Simulating SUL2 behavior on a single-instance development machine" per https://www.mediawiki.org/wiki/Extension:CentralAuth ; I had not noticed this error until now.

The notice is now on line 833 in method attemptAutoMigration, where it is logging

"Singleton migration for '$this->mName' on " . $this->mHomeWiki );

It looks like attemptAutoMigration() should be setting mHomeWiki to chooseHomeWiki(), but I put a debug print in the latter and it wasn't called before PHP warns.

The debug.log "[CentralAuth] Attaching local user Foo bar@testwiki by 'primary'" always has an '@testwiki' in it, so seems to figure out the user's wiki fine. "Just annoying" indeed.

Change 80325 had a related patch set uploaded by Mattflaschen:
Cache a variable locally before it is reset on the instance

https://gerrit.wikimedia.org/r/80325

Change 80325 merged by jenkins-bot:
Cache a variable locally before it is reset on the instance

https://gerrit.wikimedia.org/r/80325

This is no longer showing up on my test install, so I think it's fixed