Ah, I understand what you're talking about and unfortunately that's also a coding issue. The coding works by looking at the beginning [start] tags and then looking at the next tag to see the [/end] tag.

Eg:
[b][i][u]Test[/u][/i][/b]
[code][b][i][u]Test[/u][/i][/b][/code]

Easiest way to imagine it is like maths. First you look at items within the brackets, then the multiplication/division, then the addition/subtraction.
10+4x(9-7)=18
Doing it out of order and let's say from left to right and ignoring the brackets could end up with 119, which is a wrong answer except with coding it ends up with formatting, unrecognisable errors.

So let's say you have:
[code][left][right]text[/left][/right][/code]
Is that text meant to be left aligned or right? or because it's both should it be centered? Maybe it should spread from left to right. The program just simply doesn't know because it's only told to follow a simple command that has resulted in two or more answers.

So in essence what you are trying to do is not center a text part, but trying to center a formatting element which has no formatting properties to begin with as it is there to serve the text, much like [b]Bold[/b], [i]Itallics[/i] or [u]Underlines[/u]. Treat everything within a hider as an entire entity or a grouped selection of text. When you tell the hider to center and the interior text to left align you are actually saying, "I want you to do this, but I also want you to do that. Now I know you can only do one thing but I want you to do both at once."