The semantics of "who" and "what"

I think you can see the difference between these:

1 I don’t know who an actress is.

2 I don’t know who is an actress.

The difference is pretty clear. But what happens when we use the word WHAT? Does the difference remain or not?

3 I don’t know what a problem is.

4 I don’t know what is a problem.

Again, the difference is clear. At least, I see it this way:

3 I have no idea what is the thing called a problem.

4 I have no idea which thing of these is a problem.

But why do natives say that these mean the same?

5 I don’t know what the matter is.

6 I don’t know what is the matter.

Technically, they should mean different things:

5 I have no idea what is the thing called the matter.

6 I have no idea which thing of these is the matter.

While 1,2,3 and 4 can be easily interpreted and show the difference in meaning easily, 5 and 6 are resisting to be interpreted with the difference I mention, though, technically possible. What’s the matter with them? Where does the problem lie? In the semantics of WHO and WHAT or in the semantics of the article A and THE or the word MATTER? WHat do you think?