The third description-setter option is "Regular expression for failed builds". The help says
"If set, this regular expression will be used instead of the regular regular expression when the build has failed."

So I left this field blank expecting the set Regular Expression to be applied to all runs. But it turns out it is only set for Successful runs.

Filling out the failed regrex works and the description gets properly set for failed runs.

This bug can be fixed by updating the help or making the behavior match the existing help.

I prefer updating the behavior. If someone does not want descriptions set for failed builds they can set the field to an empty string. Or maybe have a checkbox so you can choose setting description of failed builds.

Nice plugin by the way.