~org-src-font-lock-fontify-block~ modifies ~match-data~ through the fontification of the temporary source buffer. But ~org-src-font-lock-fontify-block~ is also called in ~org-fontify-meta-lines-and-blocks-1~ by ~font-lock-fontify-region~. There it puts the text property ~font-lock-multiline~ on some text from the beginning up to the end of the last match in the Org buffer. Since the source buffer is smaller than the Org buffer ~match-beginning~ is smaller than it should be.
This can slow down editing operations in org-mode with large source blocks to an extent to which org-mode becomes unusable. An easy workaround is: #+begin_src emacs-lisp (defun org+-with-save-match-data (fun &rest args) "Run FUN with ARGS but save `match-data'." (save-match-data (apply fun args))) (advice-add 'org-src-font-lock-fontify-block :around #'org+-with-save-match-data) #+end_src Emacs : GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, cairo version 1.16.0) of 2020-09-19 Package: Org mode version 9.3.7 (9.3.7-4-gba6ca7-elpaplus @ mixed installation! /mnt/c/Users/toz/Weiterbildung/Soft/Emacs/ and /mnt/c/Users/toz/.emacs.d/elpa/org-plus-contrib-20200615/)