I had the same problem with my iPhone 7, and might have found a hack to solve this.
- Make sure you are connected to a WiFi network
- Open Photos and "Moments"
- Go into browse mode where you can see years.
- Go into the "Year" by clicking the > arrow.
- Scroll until you find grey photos.
On my phone this seem to have asked for the thumbnail. It also seems like it was present on the phone without being rendered, as a whole page of thumbnails were populated in just a moment.
- Continue scrolling down until you reach the end of the year.
Do this just as fast as you can see the pictures turn from grey to actual thumbnails.
- Go out and leave the phone on the selected year.
Pictures here will still be gray, but thumbs will populate after some time. Leave the phone active, as it will take a while.
- If some pics still won't show, just do it again.
- Next year / repeat procedure.