Thank you for the detailed description. I was able to reproduce the issue, but I don't believe it's related to MSTest.
When you rename a test that is used as a Test as Step, the actual file name changes and the Test as Step reference is visually updated in the Test as Step's description within the master test. The change seems to still be "in memory," however, and not persisted to disk in association with the master test. If you run MSTest at this point, the master test will still point to the old file name.
To update the Test as Step reference, the master test must be saved so the change "takes." You may need to create a Solution file for the project so you can save after a file renaming. When I ran MSTest after saving, it found the sub-test and executed successfully.
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