Okay, I can't believe I missed this, but the plugin(s) I was developing were disabled in the preferences panel of the test instance of IntelliJ. I was able to enable the plugins in the preferences and it started working again (and they continue to work because I assume those settings are persisted either in the test project or the development sandbox settings).
I still have no idea how I disabled them in the first place, and I can't believe I didn't see that they were disabled in the preferences pane...