Take a look at this example:
I selected a bold word but that is not what I got.
Why? Because under the hood the original looks like before **Bold sentence with words.** after but that is not displayed to the user doing the quoting. They saw rich text, selected a bold word in rich text, and expected it to come out bold. They didn’t see themselves as selecting a section of markdown text that didn’t include asterisks.
Also, observe this:
You call this the software not changing the selection and just being literal. But these results only make sense in terms of rich text. In terms of the underlying markdown, the software changed it on me. My selection didn’t include the closing **, but the forum software added that in.
If you want people to quote from raw markdown, display it to them and let them select it directly to quote, I guess. Having them select rich text but get quotes (partly) based on markdown is confusing and leads to results users don’t intend.