I guess it's just a display bug, then? Though it's hard to imagine what kind of bug would lead the game state and the visual representation to get out of sync in that particular way.
My guess is they only remove captured pieces on the moved-to square (maybe relying on an implicit capture by overwriting an array entry). This is probably easier than actually tracking pieces that get captured.
Are you sure? Do you mind linking to information / reporting about that? I have not seen any.
https://en.wikipedia.org/wiki/Deaths,_detentions_and_deporta...
Then you can read the congressional report:
https://www.congress.gov/119/meeting/house/118180/documents/...
At this point this is not an accident it's an intentional policy to spread fear and suppress dissent