This should not be the case; users should be able to open Implemented designs with the part that it was implemented with.
This issue has been fixed in 13.3. However, the fix required capturing additional information in the run area. That information needs to captured when the run is launched, hence older runs (that were already *launched* with the old code) lack this information. If the run is re-launched in PlanAhead 13.3 or later, it should then capture this info correctly. All newly launched runs will also work correctly.