ThumbnailCamera is used by the Roblox website to allow developers to set a custom camera position for their model thumbnails. It is not respected by the model upload window however.
In this screenshot I have positioned the camera where I’m happy with it, and added it to the model as a ThumbnailCamera. Then In the upload window you can see that the preview totally disregards what I’ve set up.
Update: This bug is still in middle of being fixed. Not sure as to the time line for when the fix will be released but when i get more info i will pass it on.
Was a flag reversed on this? Looks like it’s still using the old behavior. In the model is a duplicate Camera instance from workspace renamed to ThumbnailCamera.