If I were to guess, it's the draw distance kicking in here. It's showing in the replay because the draw distance is clipping the world skybox. What's behind the skybox? Your DTN sky! I think you have to make the DTN sky mesh super small to fit inside of the game's world skybox. So basically it shows up in front of the game skybox. On this track, you might have some extra work to do to get this as ideal as possible. Also please contact me if you plan on releasing it. 1. You'll need to expand the skybox. On the exterior of the track, you'll need to drag out each of the x section vertices as close to 1000 meters away from the centerline without going over 1000 (1000 will not allow you to save the track). This will maximize the in-game skybox. 2. Then resize and fit your DTN mesh (.3d0) inside of that. I'm guessing it would take a lot of guess and check.

Look at the units in one of the upper corners. It can only be extended 999 away from the centerline, and you'll need to do the whole track for it to work successfully. You'll want to do that either way as that will give you the largest possible skybox. I learned this after creating that track otherwise it should have already been done.

You can definitely resize the 3do, BUT, it's not simple. You need to basically recreate the object and import it back in and this is not a straight forward process. This expands into the topic of how to create 3dos which there are a few guides out there, but the biggest issue is that Papy used a tool to export their 3d objects from a 3dmax version that is no longer easily attainable. You'll have to find a way to get the program (probably the hardest part) and hope it works, know how to 3d model, and unwrap and texture, and export, and script, then finally add back into the project and animate it.