This issue seems to be focused on one machine, but a DXF created in SWx of a flat pattern ends up with geometry in DS that cannot be selected. It acts like a frozen layer but there is only one layer in the file. I tried it on a second machine and it worked just fine. Both are running SW 2018 SP2 and DS 2018 SP2. It seems like it would be a bug in DS, and reinstalling DS on the affected machine doesn't fix the issue.