Something like this? (From Chatgpt, it seems to work):
;;;_ Remove the "\\" automatically inserted at the end of the 1st line of any
Org log note
(defun my-org--strip-trailing-backslashes-in-first-line (orig-fn &rest args)
"Around advice for ORIG-FN that removes a trailing '\\\\' at the end
of the first line inserted by Org log notes (e.g. `org-add-note`)."
(let (min-pos max-pos)
(cl-labels
((tracker (beg end &rest _)
;; Record the full region modified by ORIG-FN in this buffer.
(setq min-pos (if min-pos (min min-pos beg) beg))
(setq max-pos (if max-pos (max max-pos end) end))))
(unwind-protect
(progn
(add-hook 'after-change-functions #'tracker nil t) ; buffer-local
(prog1 (apply orig-fn args)
(when (and min-pos max-pos (< min-pos max-pos))
(save-excursion
(save-restriction
(narrow-to-region min-pos max-pos)
(goto-char (point-min))
(end-of-line)
;; If the first inserted line ends with "\\", remove it.
(when (looking-back "\\\\\\\\[ \t]*"
(line-beginning-position))
(replace-match "")))))))
(remove-hook 'after-change-functions #'tracker t)))))
;; Activate the advice for Org functions that insert log notes
(with-eval-after-load 'org
(when (fboundp 'org-add-log-note)
(advice-add 'org-add-log-note :around
#'my-org--strip-trailing-backslashes-in-first-line))
(when (fboundp 'org-store-log-note) ;; some Org versions use this
(advice-add 'org-store-log-note :around
#'my-org--strip-trailing-backslashes-in-first-line)))
