I just tried replicating this with my theme (Edge) and it’s working fine with pasted text.
What theme are you using?
If you put “- Here is some text” into a markdown field in Ghost, you’ll get the hyphen turned into a bullet point. But if you just paste it into the editor, it’s normal for me…
If you’re pasting text that is formatted like markdown then it will be automatically converted to the equivalent rich-text. To avoid auto formatting then you can paste with Ctrl/Cmd+Shift+v.