We've see reports of pictures going missing from the Hidden album, but there's not enough information to imply any sort of pattern, or to indicate why or where. Your best bet is to restore your pictures from your own backups.
If you don't use iCloud Photos, but you have Settings>Apple ID>iCloud Backup turned on, then your other option is that you may be able to use that iCloud Backup to recover data from the last 30 days or so, but you would need to look at the date of the backup to be sure it's from before you lost the images. You should look here:
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
This is what you do when you buy a new phone, so this safety, if you have it set, requires you to erase everything on your phone and replace it with the backup.
If you don't keep your own backups to an external drive, then there really isn't any place else to look.