When handing other peoples’ markup, like on the Micro.blog timeline, one wants to be very restrictive with what kind of elements and attributes to allow. Partly not to mess with layout, but also for safety reasons. The approach most take is to strip away all HTML and then carefully add some elements and attributes back using an allowlist.
So I’m guessing it’s not a bug nor a feature, it’s just a consequence of the fact that @manton is pretty restrictive with what HTML is allowed on the timeline. title hasn’t made it to the allowlist for images (but it is allowed for links).
The title attribute’s main use case is to describe iframe elements. It should “contain text representing advisory information related to the element it belongs to”. It’s allowed to use title attributes on images, but it often doesn’t make much sense. What kind of advisory information is needed for the image?
If you want to have a caption for the image, it should go in a paragraph close to the image or, if you want to be real fancy, use the figcaption element.
The title attribute comes with accessibility concerns, so if you can find other solutions to what you want to accomplish, more netizens out there will be able to access what you share.