I've had this same issue. You actually have to go into the app, and select "My Page". Then select the gear icon in the top right corner to access the app settings. And then select "Clear Downloaded Data". This will wipe the old thumbnail from your device's cache and load the new thumbnail in the app.
I made an AR scene and published it. At the time, the system auto-generated the thumbnail that appears in the mobile app.
I then made some changes and published the same scene again. I also made sure to position the camera in the studio app to make sure to create a new thumbnail. However, when I look at the scene in the mobile app, the thumbnail is not updating even though the scene updates when I actually load and view the scene. Is this a bug?