Sometimes parts of a model are assemblies themselves, and when you split to objects all the parts drop flat onto the bed and look different. Try add a cube primitive to the project. Then before splitting the model you want a piece of, go the object list and find the bits you want and copy them. Now select the cube and paste the parts into the cube. Then delete the cube from the assembly. FYI, if ‘split to parts’ is not enabled it means the model is already an assembly of parts.
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