Unexpected fill-paragraph behaviour in message-mode

[Ryan Kavanagh <rak@rak.ac>]

Hi,

I am encountering unexpected behaviour when trying to fill paragraphs in
message-mode. In particular, fill-paragraph joins paragraphs (despite
separating new lines) or inserts unwanted tabs. How can I make
fill-paragraph behave as in other text modes?

Below is a small working example for emacs 28.2 that works when loaded
with `emacs -Q`.

Thanks,
Ryan

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Subject: Weird fill-paragraph behaviour
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Emacs has weird wrapping behaviour in message mode. To reproduce,
first (M-x message-mode).

Filling this long line (M-q anywhere on it) joins it with the following indented text, which it should not because there is an empty line marking the end of this paragraph:

    Lorem ipsum dolor sit amet, consectetur adipiscing elit.
    Sed quis iaculis erat, at placerat ligula.

Moreover, if I have a long line in a paragraph following indented
text, then attempting to fill it (M-q) causes emacs to insert indents.
Indeed, place your cursor anywhere on the following line, and then hit M-q
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris a massa pulvinar, lobortis massa at, viverra massa. Suspendisse fringilla malesuada erat. Curabitur eu dui lorem. Aenean tincidunt, leo eget tincidunt tincidunt, urna leo mattis eros, in vestibulum mi purus in nibh. Fusce sem elit, vulputate ut tristique a, ornare vitae mi. Quisque sed turpis in metus facilisis mollis et sed augue. Nam vel efficitur erat, id maximus eros. Vestibulum interdum nisl id suscipit sagittis.