mmm not a bug as such, I would think, but rather a configuration issue. Where and how, I have no idea but it is likely to be that someone has done something in code, or reset something or edited part of the editors management page, could have been anything....
Time is an issue when trying to isolate this kind of issue. How much time are you prepared to spend hunting down the problem then resolving it?
There is two alternatives here. One is you extract from the same version download the entire editors folder. You then go into the codebase, delete the entire editors folder and all the editors and using the extracted editors folder paste that itom the Moodle root.
This is not a Moodle recommended method, btw, but it is one I have had success with when admining smaller, private, Moodles. If there is a problem in a single module, replacing that module seems to have a positive impact. Won't say it will in this case, but this is quicker and less messy than the alternative.
The other alternative is to replace the entire codebase. You can update your site or you can use the same version you now run. Keep the current config.php, delete all other code and replace it with new code, overwrite the new config.php with the older one. Make sure you have complete backups of both the database and the moodledata folder.
One of these may be quicker than fixing an issue that you may not get to the base of. If the first doesn't work, then the second becomes the obvious response.