In doing some analysis for making the main stash DC-aware, I came across this code in the OAuth extension:
It's explicitly getting the session storage using $wgSessionCacheType and using that for writing its own data. This will probably cause some problems with the test roll out.
I think the right fix here is to change it to use its own configuration variable for its storage backend, and fall back to MediaWikiServices::getInstance()->getMainObjectStash();
As far as I can tell, none of the 176 other extensions we use in production use the SessionCacheType config variable explicitly, and it also doesn't appear in unexpected places in MediaWiki core.