It's not a bug. The automatic addition of a title attribute/tag to image markup was deliberately removed in WordPress 3.5 as it represented an accessibility issue. Tooltips can still be added by opening the Edit Image modal and inserting something into the title attribute box. The only change is that the "Title" field — which is the name of an image's attachment page — does not populate the title attribute.
Screen reader, switch, VR users and sighted keyboard navigators do not have access to the title attribute, therefore any information that is being presented via that attribute is hidden from them. Site authors should NEVER rely on that attribute to convey information that, ideally, should be presented in clear text on the page. This contravenes the first of the 4 principles of web accessibility - that content should be Perceivable. WordPress core is now merely following best practice in this regard by not auto-inserting non-perceivable content.
If something is worth putting in a title attribute, then 9 times out of 10, it should be on the page instead - where everyone can see it.