I'm using message-mode right now in Gnus to compose this message. I
just typed this paragraph below and I pressed M-q to fill-paragraph and
it works as expected.

--8<---------------cut here---------------start------------->8---
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim
veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea
commodo consequat. Duis aute irure dolor in reprehenderit in voluptate
velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint
occaecat cupidatat non proident, sunt in culpa qui officia deserunt
mollit anim id est laborum.
--8<---------------cut here---------------end--------------->8---

Now, I often use message-mode outside of Gnus. For instance, sometimes
I might want to produce some text files that would look like e-mail
replies. So I paste the text in *scratch*, say, I turn on message-mode
and I expected EMACS to behave just it does when I'm here in Gnus. This
has been my experience, for example, in GNU EMACS 24.3.

However, consider this experiment. Take a long line, paste it in
*scratch*, change the mode to message-mode and try to fill-paragrah on
the line. For instance, you can use this long line:

--8<---------------cut here---------------start------------->8---
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore [...]
--8<---------------cut here---------------end--------------->8---

If you're using GNU EMACS 27.1, you'll end up with

--8<---------------cut here---------------start------------->8---
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor
incididunt ut labore et dolore [...]
--8<---------------cut here---------------end--------------->8---

In fact, if the line were longer, you'd end up with this indentation.
To work around this, I take message.el from GNU EMACS 24.3, I
eval-buffer it and then fill-paragraph on the *scratch* buffer and it
behaves as I expected it to.

--8<---------------cut here---------------start------------->8---
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
tempor incididunt ut labore et dolore [...]
--8<---------------cut here---------------end--------------->8---

In other words, from my perspective, message.el in GNU EMACS 27.1 has
introduced programming errors that do not show up when in Gnus. So far
I have not identified which procedure is the culprit. I wonder if
anyone knows anything about it.

(*) Version

GNU Emacs 27.1 (build 1, x86_64-w64-mingw32) of 2020-08-21