I see this regression using 25.1 (tarball) and emacs-26.0.50 (git).

Reverting

  http://git.savannah.gnu.org/cgit/emacs.git/commit/lisp/vc/vc-cvs.el?id=fa7f79e8234c60ae425f7c3cf1b9486765a7111e

fixes the problem here.

-- 
jca | PGP: 0x1524E7EE / 5135 92C1 AD36 5293 2BDF  DDCC 0DFA 74AE 1524 E7EE
OpenBSD emacs port maintainer