it wouldn’t surprise me that spam filtering is blocking the message. I just tried this on iPadOS 26, and the message also disappeared. I do need to check a few other details about the recipient mail first though, such as sufficient mail storage quota.
Add some text to the message, and use a subject line, and test again.
There are other ways to send photos, including using a link into your iCloud Photos storage.
Or if you’re moving these photos among your own gear, iCloud Photos can be useful, as can AirDrop and some other options for transferring images.
Sending these photos to yourself, or to others?
Are you already using iCloud Photos?
(With these answers, there might be an alternative for making the photos available.)
Unfortunately for this current case, mail is not entirely reliable, and will probably only continue to get less reliable. It should work, but it sometimes doesn’t.