SW has poor associativity with its own multi-body parts. This seems to be causing issues in DM (body count increases or decreases with each update, Named selections become invalid, etc.)
In other words, suppose you have a multi-body part in SW and attached it to DM. In DM, if you give named selections for some bodies, then refresh the model in DM (Use Geometry Parameter values), the named selections for bodies can be mixed up. They may point to different body/bodies ' even if you did not change any parameter in SW. If you have some body operations later in DM tree, you have to revisit them to make sure they are acting on the correct bodies.
Development has known the issue and commented that: The associativity of SW bodies, in multi-body parts, is a known and long-standing issue. They have tried three methods with the SW API and have either hit road-blocks or achieved limited functionality. If the model were an assembly instead of a multi-body part, the expectation would be the named selection would behave as expected. They are working toward resolving this issue for v12 but at present it is a limitation of SW. They would not expect the same problem with ANY other CAD system.
I tested with SW assembly, and the associativity was indeed preservedin DM. The assembly has to be a multi-part assembly, in other words, take the multi-body part and convert it directly (From File > Make Assembly from Part) to a single part assembly won`t work.
Here is how to create multi-part assembly:
Go to each body in the SW tree and right click. Select "Insert Into New Part" to create a part from the body. When all bodies are completed, create a new assembly document and select one of the parts as the base model (I don't think it matters which). After that, there is an "add component(s)" menu item. Then just add each of the parts, one by one, into the assembly. Since the exports already had the transforms built in there was no need for assembly constraints. Also, I think the part models referred back to the original multi-body file since when one was updated the others "knew" of the changes and asked to be updated as well.
Please also note that unfortunately in this way all the parameters in your original multi-body part will not be presented in DM. The problem there is that the parts do not have parameters directly and they were generated from the model that has all the sketch and parameter details. If the model was created from scratch as parts and assembled the parameters would be there.