>>625
smartparens (と Emacs 26 の CC Mode の組み合せ) に由来する症状らしい

Single quotes failed in c/c++/php mode since Emacs 26 HEAD-59d0787 · Issue #783 · Fuco1/smartparens · GitHub
https://github.com/Fuco1/smartparens/issues/783

当面の回避策
(add-hook 'c-mode-common-hook (lambda () (setq sp-escape-quotes-after-insert nil)))