The company quietly improved this feature with One UI 2.5. Samsung's Motion photos now capture both video and audio, as opposed to only video in the past. You might want to take note if you back up your content to Google Photos. It seems to be having a problem saving Motion photos properly.
Motion photos being stripped of the audio and video
According to multiple reports, Motion photos being backed up to Google Photos are only being stored as still images because the service is stripping them of the video and audio. This seems to be a recent change because Google Photos was properly backing up Motion photos with the audio and video up until last month, even though the processing took a long time.
Motion photos may have been around since 2016 but Google only started supporting them in Photos last year. Clearly, this isn't something that the company attaches a lot of importance to. It's likely that a recent update to Google Photos has broken its ability to properly save Motion photos.
Google will need to look into what happened before it can go ahead and fix this bug. As for when it will get around to doing that is anybody's guess right now. So if you don't want to lose them, it would be best to keep copies of your Motion photos on the device as well.
