Update: This worked for just under a week. I've resorted to streaming the photos directly from my phone for the time being.
(originally) I figured this out, this morning, at least for me. I'll post here in case it helps anyone else.
Mine worked fine until about the first or second week in November.
The Google Photo Album must have 'Link Sharing' turned on. I'm not sure what changed from the time that I set this up to when it failed, but I turned on the 'Link Sharing' feature on one album, and suddenly they all populated again in the Roku Backdrops site.
To be more specific, I went into the Google Photos Album online at the Google Photos site. I clicked in the top right corner of the album and selected 'options'. I turned on link sharing, but unchecked a couple of boxes about other people adding photos and whatnot. I did NOT need to share the link with anyone for this to work.
On the Roku webpage (where the Backdrops QR code takes you) the images were suddenly visible again.
On the Roku TV, I had to go in and out of Backdrops once for it to repopulate with the albums. I had unlinked them previously as I was troubleshooting.
