Build job was executed in Release mode and failed because of unit test failure but 
action specified in "After successful release build" was executed.
It looks like action specified in "After successful release build" was executed before  check for unit test failure was executed in "Post-build Actions"-related "Publish xUnit test result report".
It is possible to analyze unit test result before "Post-build Actions", by adding to build steps "Process xUnit test result report" but such approach generates warning that "XUnitBuilder step is deprecated since 2.x, it has been replaced by XUnitPublisher. This builer will be remove in version 3.x".

Problem is that XUnitPublisher ("Publish xUnit test result report") only available  as post-build step!