How to convert a gallery story to a video on Galaxy S8+?

This is what I know.

Try to add more photos (at least 6 or more) to your story, then you should see a Play button in the lower right corner of the story cover.

Press the Play button play the video. You can also do some editing here. Once you’ve done with the editing, you can tap Save to generate a mp4 video.

I have the same problem, but I had a story automatically created by gallery and had the play button. I remover 3 pictures, leaving 18 pictures in total, in the story, and the play button disappeared. How can I bring back the play button?

I have the same problem but can't find any answers for a fix.