I've been following the Build a Java app with Maven tutorial and the soon-to-be-released "Build a Python app with PyInstaller" tutorial (whose application source repository is here and whose Jenkinsfile I'm using to build it is here.

In the Java app with Maven case (see attachments beginning with 1 and 3), the Test results show up fine in the classic UI and Blue Ocean.

In the Python app with PyInstaller case (see attachments beginning with 2 and 4), the Test results show up fine in the classic UI but not in Blue Ocean.

Update:

This only appears to be a problem the first time you run a Pipeline (with a valid junit step). Subsequent runs and re-runs are fine and give you the results depicted in the attachment beginning with 5.