Reporting back - I launched the Reason 12 DAW and received a message that some rack extensions were out of sync and that there was an update available. I was also required to sign in again, then sync’d the racks, installed the update.
Relaunching GP, I was able to build a new rackspace with the Reason Rack plugin and everything worked as expected. Next I opened the original song/rackspace that was giving me trouble and it was functioning as well as the day I created it.
Based on this, my guess is that Reason needed me to login again but there is doesn’t appear to be a mechanism to prompt that user to that action from withing the Rack plugin. Of course, there may have been a bug that needed to be address by the software update. In any case, following these steps resolved this curious issue for me.