When a craft gets within loading range of another craft, the other craft would continuously teleport a few dozen meters away from the craft you are controlling. Quicksaving and Quickloading caused the trajectory lines to disappear and for the craft to stay in position with 0 velocity relative to kerbin.
This is in the latest patch with an AMD Ryzen 5 5600X 6-core, 16GB RAM, and an RTX3060 GPU.