fill-individual-varying-indent

fill-individual-varying-indent is a variable defined in `fill.el'.
Its value is

nil

Documentation:
Controls criterion for a new paragraph in `fill-individual-paragraphs'.
Non-nil means changing indent doesn't end a paragraph.
That mode can handle paragraphs with extra indentation on the first line,
but it requires separator lines between paragraphs.
A value of nil means that any change in indentation starts a new paragraph.

You can customize this variable.