Dolphin generates previews for video files through ffmpegthumbnailer; but it also generates a few sequence images from the video when the mouse is hovered over it

what is this feature called?

I want to troubleshoot why some video files are getting the thumbnails generated, but not the sequence of images, and have found it quite diifcult finding something specific to this feature - all sorts of problems solved seem to be related to the thumbnail system itself, not the sequence of images