I've had this problem since the launch of Trackmania 2020 where there are no car shadows in any replay, the replay editor and therefore rendering.
See Example:
There are a few threads that suggest fixes for this problem.
https://www.reddit.com/r/TrackMania/comments/iocovj/car_sadows_disappeared_from_replays/
https://www.reddit.com/r/TrackMania/comments/qlvb1l/car_shadows_are_missing_in_replay_editor_and/
https://www.reddit.com/r/TrackMania/comments/q995dn/car_shadow_does_not_show_in_rendered_replay/
https://steamcommunity.com/app/375900/discussions/0/392184522710668791/
https://forums.ubisoft.com/showthread.php/2266675?pp=40: (https://forums.ubisoft.com/showthread.php/2266675?pp=40)
I've tried all of them to no avail. It seems that only a very small subset of players have had this issue continuously. The last one is a cloesd thread from the official forums but that does not solve the issue for me.
My GPU is a RTX 2080 (Latest Drivers) and all Settings but reflections are maxed out ingame.