How to save Daz Scene Subset with Iray render Thumbnails image?

One day, I saved a file as Scene Subset (while in Rendering preview mode), and unexpectedly, it was saved with Thumbnail image exactly like the Rendering preview display. And I did it with many other objects and characters.
Then, when I opened my computer the following day, I couldn't do it anymore. My Thumbnail images became Shader preview image, like any normal day, although I had turned it to Rendering preview mode while saving it.
I hope someone can give me some advice :D
Comments
Not sure how you could save DUF thumbnails as previewed image... as default, DS just saves DUF files with your current Viewport by using OpenGL.
I don't know, but it suddenly appear.
That's interesting ! What's your DS version ?
4.22
Well, I had no way to reproduce this "nice feature" in both 4.22 / 4.23. And I never saw and could make this result before... see if other folks can find it out ~~