Image in email attachment is not saving to the Photos app
I would imagine it's the same story as saving a screenshot or any other photo from online etc.

Pictures like that are not automatically added to the Photos app, you have to import them yourself.

You can do this from the Photos app: File -> Import then select whichever ones you want. It's handy if you always save them to the same folder(maybe Downloads or a handy custom folder on your desktop).

In regards to why some of them saved but others didn't, I'm not even sure why some of them did save in the first place.