Create a suite with a single test case with no steps. If you export that, it will have a TestPointId without a colon (e.g. - 170). You can add results and import it successfully without steps.
If you then modify the test case to add test steps, the TestPlanId will be updated to something like 170:0 indicating it has test steps. You can import and from the new excel file successfully.
If you then remove test steps from the test case, you can no longer import results -- even if you are using the latest exported Excel file. It continues to look for test steps based on the TestPointId which still appears as 170:0.
Basically, if you ever add test steps and then remove them for whatever reason, offline testing breaks that test case from import from that point forward.