This is Not a Bug. In LaTeX, A_bc will only treat b as a subscript. In order to include multiple symbols in a subscript (or superscript) you need to group them with curly braces.
For instance, here is A_bc: while here is A_{bc}: .
Thanks for your answer, but that is not the problem. Please, look at the code and the results: you will see that no subscript is displayed, nor even the first one (an underscore is displayed instead). So, yes, it is a bug of the Control System Toolbox.
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