I beleive what is happening here is that your custom object might have been created with coordinates far from the origin before you imported it into Twinmotion. Twinmotion uses the coordinates information inside your model to determine where the object is displayed after import. If you create an object far from the origin in your source program (Sketchup, Archicad, etc.), then that coordinate info will be preserved after you import it. If you move the model closer to the origin in the source program, that should fix this issue.