video thumbnail is reset in iCloud Photos and often unidentifiable

Since iOS 13 the thumbnails for videos in the Photos app (with iCloud on) are reset overnight. When first saved the thumbnail shows the first frame of the video but after overnight backup they instead show a frame from anywhere in the video.

I assume this is an iCloud routine to automatically assign a distinct thumbnail but nearly all my saved videos start with an obvious title and this is usually replaced with an often unidentifiable scene. Even videos made several years ago are affected.

My only solution is to copy the videos to the Files app on device which restores the first frame thumbnail (and also allows file naming and search). Still I’d like to be able to use Photos features such as the location map with recognisable thumbnails.

I am surprised that searching doesn’t show any mention of this issue. Am I the only one? I’d appreciate any help or just information. Am I right that the thumbnails are reset using some sort of AI? How does it work? Is there any way of assigning a thumbnail manually?All the above videos started with a large title identifying the place. Some of the new thumbnails could be anywhere.