I believe your latest suspicion is probably correct. I found the same thing that you did if I selected when Focus Highlight showed me to be in browse mode with a thin, green dashed line indicating the navigator object selected. When this was text of any kind I'd get straight text - formatting was not retained.
I used the methods I document here:
once I've got myself positioned where I want to be, or at least roughly so, I really don't know how screen reader users ever get precise positioning in masses of text easily, as I'm certainly crappy at doing so using only the keyboard.
If I select that way all formatting, and additional content if the text is interrupted by an inline image, is retained and pastes perfectly.
Someone else who's a better NVDA user than I am can test the details out.
Brian - Windows 10 Home, 64-Bit, Version 1803, Build 17134
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