WordPress 3.5 introduced a new feature – when images are inserted into posts, the title attribute is not included in the image tag. This was done with the best of intentions relating to accessibility, as documented on Trac
Unfortunately, this causes problems for some Lightbox plugins, quite apart from stopping image tooltips from appearing.
This plugin hooks into the media_send_to_editor filter and inserts the image title into the html inserted into the post.
The original author, LesBessant, sadly passed away and in the meantime the original plugin was closed. I plan to keep the plugin up to date whenever an update breaks it.