Motion photos not recognized by Google Photos in shared albums

After adding a motion photo into a shared album, Google Photos sometimes fails to show the triangular ▶ play button.

Right from the horse’s mouth:

Google Photos can be a bit buggy with Live Photos in shared albums - the best way I’ve found to combat this is to just upload to Google Photos first (don’t add them to a shared album just yet), wait a bit (can be anywhere between a few minutes to a few hours), then after some time, add them to the shared album.

You can also check if the Live Photo is fully processed by going on in a web browser, and clicking on the photo - if the play button icon is fully white, the motion will be there; if the icon is faded, it’s still processing.

Reddit thread

The solution is simple: wait for the photo to finish processing (the triangular ▶ play button starts working), and then add it to the shared album.

It’s also interesting to note that this could affect photos shared via the “Partner sharing” feature. YMMV.