Finally discovered a solution, as per this thread: https://forums.unrealengine.com/t/gauntlet-test-unable-to-find-test-in-assemblies-5-4/1920436
Since the project is outside of the engine, it requires adding -scriptdir= (the folder containing the .csproj)
For example: -scriptdir=D:\Unreal_Projects\Lyra\Build\Scripts