I've come across this with Convenient-Discussions trying to delete a property created for testing purposes, and I couldn't. I was lucky not to mess with the users' options (only faced this behavior while in the development stage)—they would have an undeletable junk option. A particularly unfortunate feature of this is that global preferences override local ones, so if there is a userjs- option with the same name locally, it would have no effect.

Note that this task is a duplicate of T207448: Resetting a custom global preference via API does not work.

@Jack_who_built_the_house: Feel free to Edit Related Tasks... → Close As Duplicate in the upper right corner in such cases. Thanks!