Hi ,
Discussing with the team, what is happening is that on export, Twinmotion is hitting an LOD limit. In the standard raster rendering mode, there is a limit to the number of high details LODs that are able to be rendered at once, so the tress closest to the foreground are prioritized for high detail rendering, while trees in the background are rendered with lower detail LODs. This limit is in place to prevent crashes during the export process.
If your graphics cards support Path Tracer, this limitation is not present in Path Tracer renders, so all trees will be rendered with maximum detail.
If Path Tracer isn't an option, then while it won't be possible to render all trees with high detail LODs, the final image can be greatly improved by making sure all trees in the background are placed on a surface using a grass material, which will allow the trees to blend more naturally with their surroundings. Also, increasing the Shadow render distance in the Render properties will make sure that the trees cast shadows, which will make them appear much less flat and mask the low details.
I hope this helps.
Best,
Raghib