Nested Categories Merging with Main Categories!

This problem is blowing my little mind.

On ONE of my many wp blogs, on ONE particular category, I cannot create a nested (sub) category without that category including in it the name of the parent category!

Even on the same blog, all of my other categories can create nested categories just fine, such as:

Math > Algebra Slug: “algebra”

…but this one is insane. I can even delete them all and re-create them again, and I still get this problem, which would look like this:

Math > Math Geometry Slug: “math-geometry”

So my permalink of /%category%/%postname% looks like this:

https://www.domain.com/math/math-geometry

Although I only typed in “Geometry” when I created the subcategory.

Does anyone have a clue what in the heck this could be?

Thanks in advance.