The fix is in this commit: 0e83bc1
It shows how simultaneously releasing and ending the pool could have resulted in termination sequence proceeding without waiting for release sequence to complete. The symptom of this would have been an error that complains about an attempt to use a terminated backend.
Shoutout to @mikeroelens for going above and beyond to chase down the root of the issue and suggest a fix and context that led to replicating the mysterious behavior. 🥳