Hi,
if you connect via proxy user (dev[app]) and want to run the tests in the app-schema, the plugin is not working.
if you connect directly as the app-user, it works fine.
using proxy users in utplsql-cli also works fine.
So it seems to be an issue with the plugin.

Debug Information:
PL/SQL Developer Version 14.0.6 64 bit
Windows Server 2016
Oracle Client 19c