The use of the backspace ⌫ to delete photos can be confusing. It depends on where you are viewing the photos when you use ⌫.
- While viewing the photos in the library albums, created automatically for you by Photos, for example "All Photos", "Recents"; etc, the ⌫ shortcut will delete the photo from the library and all albums and save it in Recently Deleted for recovery.
- While viewing the photos in the "My Albums" section, albums you created yourself, the ⌫ shortcut will just remove the photos from the album, but the photos will stay in the library, still appear in All Photos or Recents, and not be saved in Recently Deleted, because the photos have not been deleted. In albums we created yourself, we have to use ⌘⌫ to delete photos and not just remove them from the album. And ⌘⌫ will usually save the photos in Recently Deleted.
- On photos in smart albums we cannot use the ⌫ key at all, as we cannot remove photos from smart albums. Only ⌘⌫ can be used and will delete the photos.
So, try to remember, where you have selected the photos, where you applied ⌫ - in an album you created or in a library album created by Photos? The photos may not be showing in Recently Deleted, because you only removed them from the albums, but not have been deleted from the library. Look for them in "All Photos" or "Recents".
There is one situation where deleted photos will not be saved in Recently deleted, but deleted from the library. This can happen, if you are having iCloud Photos enabled and have used up all your cloud storage. Then the photos and videos we are deleting from the library will be deleted instantly permanently, without being saved in Recently Deleted for thirty days. This is necessary, so we can instantly free our Cloud storage. Are you using iCloud Photos? Is there a problem with your cloud storage? Thsat could also explain why deleted photos are not turning up in Recently Deleted.