This actually happens because, of the topics that a problem is categorized under, only the topics without any children are listed.
For example, if you edit a problem to fall under topics "Chinese remainder theorem" and "Number theory", only "Chinese remainder theorem" will show, since "Number theory" is a parent of "Chinese remainder theorem".
Ideally there would be some sort of pop-up box appearing if you try to do the above, indicating that the problem is indeed categorized under Number theory, but that it's not being displayed as such.
This actually happens because, of the topics that a problem is categorized under, only the topics without any children are listed.
For example, if you edit a problem to fall under topics "Chinese remainder theorem" and "Number theory", only "Chinese remainder theorem" will show, since "Number theory" is a parent of "Chinese remainder theorem".
Ideally there would be some sort of pop-up box appearing if you try to do the above, indicating that the problem is indeed categorized under Number theory, but that it's not being displayed as such.