Hello, I'm not sure if this is a general issue with fill-paragraph or with the way org hides emphasis markers, but I'm facing this problem with bare emacs + org:
Steps to reproduce:
1. Start emacs -Q (no configuration)
2. Set some variables:
(setq org-pretty-entities t
org-hide-emphasis-markers t
fill-column 80)
3. Create an empty org-mode buffer with the following text:
This is a /italic sentence for a buggy/ \alpha\beta\gamma\delta\nu\epsilon
paragraph in terms of filling, see, it's not filled yet and still the line above
ends early.
So far so good, and with the previous config the emphasis markers are now hidden and the greek letters appear as unicode characters.
4. Hit M-q (either fill-paragraph or org-fill-paragraph, both have the same issue)
The first line wrongly wraps after "buggy" and leave a big empty space after that, as if the greek letters were not composed.
5. In the same buffer, compare the behaviour with this paragraph (very similar, but note the ! character after "buggy"):
This is a /italic sentence for a buggy/ ! \alpha\beta\gamma\delta\nu\epsilon
paragraph in terms of filling, see, it's not filled yet and still the line above
ends early.
When I fill this slighly different paragraph with M-q, it wraps the text in the right way and the first line is completely filled.
So the issue happens when a hidden emphasis marker is immediately followed (maybe after a space) by a composed charater like \alpha. I am also facing the same issue when the characters are composed by prettify-symbols-mode instead of org-pretty-entities, and also with other types of hidden emphasis markers. If org-hide-emphasis-markers is disabled then the issue doesn't happen.
Thanks in advance!
Emacs : GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.29, cairo version 1.17.4)
of 2021-07-09
Package: Org mode version 9.5 (9.5-b83ae59 @ /home/lucas/.emacs.d/.local/straight/build-28.0.50/org/)