To reproduce:

emacs -Q --eval '(progn (setq fit-window-to-buffer-horizontally t)
                        (set-frame-size nil 120 20)
                        (with-current-buffer-window "*scratch*" nil nil
                          (insert (make-string 20 ?x))
                          (insert (make-string 10 ?\n))
                          (insert (make-string 90 ?x))
                          (split-window-horizontally)
                          (fit-window-to-buffer)
                          (message "window width: %d" (window-width))))'

Expected result: window width: 90
Actual result: window width: 20

Patch: