Test Execute keyword test fails due to mapping issues, same keyword test is passing in Test Complete

NameMapping is stored within the project.  So, you mapped the objects in the MasterProject... that is where the mapping is stored.  If you didn't map the objects in TestProject1, that's why you're running into the object identification errors.

Now, you don't HAVE to remap everything.  You can delete the NameMapping node from TestProject1 and use the "Add -> Existing Item" to TestProject1 and point to the NameMapping file that you created in the Master Project.