Error when trying to generate for a unit test module

I'm trying to generate the documentation for our unit test module, and I am getting an error. It worked for the main module though, and it also worked for the tests a couple of years ago, so not sure what's changed.

My command is:
sourcekitten doc --module-name [ProjectName]Test -- -workspace [ProjectName].xcworkspace -scheme "[ProjectName] Dev" clean build > swiftDoc.json

And I get the following error:

Could not parse compiler arguments from `xcodebuild` output.
Please confirm that `xcodebuild` is building a Swift module.
Saved `xcodebuild` log file: /var/folders/8w/w7k_803179l5pbqzm47mxx4c0000gn/T/xcodebuild-2EEBEE83-7072-42FB-A8C6-5688FF202D93.log
Error: Failed to generate documentation

But the xcodebuild log shows "** BUILD SUCCEEDED **" as the last line.

Sourcekitten version: 0.31.1
Xcode version 13.0

Start with the sourcekitten doc command path that parses compiler arguments from xcodebuild; compare its behavior for the main module and [ProjectName]Test using the command and saved log described here. Done means documentation generation succeeds for the unit-test module and the reported parse error is addressed.