Some mathematical notation actions can't undo in a notebook

Questions
1

Hello, I am using mathematica v12.3
I am new to mathematica and staring to learn. I am writing some math notes which include text and some equations.
I know that in Text cell I can CTRL+( and start typing some equations and they would appear in mathematical notation, But sometimes if some equation is highlight and I accidentally press CTRL+( key, the equation convers to some kind of markup code. for example
x^2+9-50
would become

\!\(
  \*SuperscriptBox[\(x\), \(2\)] + 9 - 50\)

and strange thing is most of the time this actions won’t undo with CTRL Z .also the undo is greyed out in Edit tab.. I have to close the file and reopen it get it to the state it was last saved.

I want to know what is this markup code, and how can I undo this action, and why it does what it does, why only on highlighted text? on empty area it works as expected.

Thanks

2

String box representation is explained in the following section of a tech note:

CTRL+( operates on selected text, if text has been selected, creating an inline cell with the selected text as content; otherwise, it creates an empty inline cell with a placeholder, which is removed if you click elsewhere without typing content into the inline cell. BTW, I don’t get the behavior you describe when I try it; in fact, undo works for me.