You won’t get prompted on a self post, but the post should still save.
If there is an image or other file (gif, image, video) you are prompted whether you want to save it to device or save it to your profile.
But with self text posts, there is nothing to save to device so there is no prompt. it just gets saved automatically to your profile.
If you are not logged in and you are on a post that has an attachment, you can still save it to device. but if you are on a self text post, you don't get the save option.