Not a big deal, since I'm not sure how many people use this capability, but start a new Minimal project with just one image, something with no title or comment attached to it. In Tools, Preferences, Album, check Write xmp metadata to files. In the Advanced > Metadata settings, choose Include xmp metadata in generated images. Make the album.
Now go back in and add a title to the image. Take a look at List metadata, to satisfy yourself that it's there. Make the album again. You will get a new slide image, with the title embedded in it.
Now go back in and add a comment to the image, and List metadata. Make the album again. You won't get a new slide image. The comment doesn't get written out to the slide image until you do something else that triggers reprocessing of that image.
ETA: Adding a keyword also triggers the creation of a new slide image, as expected.