Is my mouse broken?

In Fortnite I have a mouse, mouse, thumb button 1 for the wall and 2 for the stairs. I have to hold down while building kch and if I build a wall and then want to build the stairs, it doesn't work (if I press the left mouse button while doing this. But it works the other way around

So my question is why I hold down the 1 to 2 key while the left mouse button is pressed.

I answer your questions lol

Better go into settings again and check it out