When using `org-fill-paragraph' on a part of a paragraph, e.g. with halve the lines as active region, it still uses the unselected text around when filling.
This works different than `fill-paragraph', `fill-region' and `fill-region-as-paragraph' that fill only the selected part of a paragraph WITHOUT effecting the text around.
I don't know if this is a real bug but it is confusing, and annoying because org-mode overrides the default `C-q' key binding for `fill-paragraph' so I have to invoke that command via M-x.

Emacs  : GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.5, cairo version 1.16.0)
 of 2021-02-09, modified by Debian
Package: Org mode version 9.3 (release_9.3 @ /usr/share/emacs/27.1/lisp/org/)