I had very similar problem with our dotnet3.1 test container and figured out that this was only happening when dotnet was specified as an entrypoint. When I changed my entrypoint to bash everything went right.
Not sure about details on why it’s happening, but this was reproducible (not always) on my windows machine
Of course maybe it’s not your case ))
And there was another guess: try to output something like “Tests completed” in the end of tests