(Not necessarily a great answer to your question, but it's in the same wheelhouse and might be handy if you're working in this area)!
I remember having issues with this myself (to be honest the precise details are a bit lost in time for me - but basically IIRC it's by design that they're not logged in after creation, perhaps to force them to practise login/their password? I can't remember but do remember I found it odd behavour) - anyway in my case the issues arose as part of adding user registration during commerce checkout. It was discussed on Slack and the solution was that Steve Rowling made a very handy plugin to auto-login newly created users:
I'm using it, works a treat.
(I should also wrap up my code for registration during checkout as a plugin and release that one of these days!)