Having exactly the same issues with Xcode Version 12.5.1 (12E507)
However, it appears to work as expected with Xcode Version 13.0 beta 5 (13A5212g)
Possible workaround could be to update manually xcscheme file as such:
<LocationScenarioReference
identifier = "{path to .gpx file}"
referenceType = "0">
</LocationScenarioReference>
Topic:
Developer Tools & Services
SubTopic:
Xcode
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