Point-Char: |

Name: fill-paragraph-handle-comment - current non-comment line before comment line
Code:
  (lambda ()
    (setq-local comment-start "# ")
    (setq-local fill-paragraph-handle-comment t)
    (setq-local fill-column 42)
    (fill-paragraph))

=-=

this is not part of the comment this is not part of the comment|
# this is a comment this is a comment this is a comment

=-=

this is not part of the comment this is
not part of the comment
# this is a comment this is a comment this is a comment

=-=-=

Name: fill-paragraph-handle-comment - non-comment line before current comment line

=-=

this is not part of the comment this is not part of the comment
# this is a comment this is a comment this is a comment|

=-=

this is not part of the comment this is not part of the comment
# this is a comment this is a comment this
# is a comment

=-=-=

Name: fill-paragraph-handle-comment - comment line before current non-comment line

=-=

# this is a comment this is a comment this is a comment
this is not part of the comment this is not part of the comment|

=-=

# this is a comment this is a comment this is a comment
this is not part of the comment this is
not part of the comment

=-=-=

Name: fill-paragraph-handle-comment - current comment line before non-comment line

=-=

# this is a comment this is a comment this is a comment|
this is not part of the comment this is not part of the comment

=-=

# this is a comment this is a comment this
# is a comment
this is not part of the comment this is not part of the comment

=-=-=