I solved this just like I did several other Moodle issues for which there were no obvious explanations.
For the record then,
Solution: I uninstalled the Assignment feedback PDF annotation plugin and just let Moodle automatically reinstall (without deleting any files). This is incredibly simple.
This deletion of its database configuration settings and resetting did the trick and cleared the bug (for bug it is).
(I made sure to back up the entire server as a precaution)