Using a plain text editor produces only plain text. Thus, the new lines are separated by carriage returns and not formed as paragraphs.
When pasted browser transforms these carriage returns to BR tags, which are not proper block elements to form a single list item. Therefore, the entire paragraph is formed to a single list item.
You can handle this scenario as a custom solution by handling the paste event
and processing the content pasted. For example, when plain text is pasted, BR tags that form new line feeds to be removed and separate lines to be wrapped in P tags. This Stackoverflow discussion might help you further with this case—http://stackoverflow.com/questions/1275250/is-there-an-easy-way-to-convert-html-with-multiple-br-tags-into-proper-surrou
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