I'm in the process of MDM joining devices for an organisation. I'm setting up photo backup on the old devices. then on the new devices I'm having to go into the me > photo's section, manually selecting all photos (you can tap - hold then drag to select multiple at a time) manually to download them. then have to navigate to the downloads folder on the phone and again select all of the photos and move them to the gallery location in order for them to show up in the app.
I would suggest you delete all files in the download folder before downloading the images this way to prevent random files showing up in the gallery after this step.
It would be a great feature to add a button to "restore gallery from backup" as this is the only thing missing from the photo backup solution. backing up the photos is great but restoring them is hard work completely ruining the purpose of having a photo backup.