I went into the DB to see the XML_DEFINITION_DATA fields for two builds.
From a build plan that's had post commands defined for a long time, back in Bamboo v5, and that runs post-build commands successfully:
<command>
<run>
<location>A</location>
</run>
</command>
From a newly-created build after Bamboo and the plugin have been upgraded (Bamboo at v6.5.0):
<command>
<run>
<location>on</location>
</run>
</command>
I think this means the plugin is broken, because the newly-created plan does not run any post-build commands.