The export image formatter can not export the title field of an image.
The formatter checks if the title field is enabled in field settings, before it offers the checkbox to render the title. But this check can not work by design: Those settings are the values of the default settings. Given that one field (.storage.yml) can be used by multipe fields (.field.yml), each with their own settings, and views fields work based on storage accross different bundles, it is not even possible to make the field settings check.
Add an image field to a node. Enable the title field on the image. Disable the alt field.
Create a rest view with that image field. The title option will not be available. The alt option will be available.
Remove the field settings check. Always offer all options in the views plugin and let the site builder decide inside the views UI.
Needs review
3.1
Code
MR always removes the image loading setting from views plugin, which makes no sense in the context of an JSON or XML export