[O] , C-c ', on SRC block in two window vertical split screen goes horizontal unexpectedly

[Van L]

Hello.

I have two windows separated by a vertical split. Left and right.

On the right window, I have an elisp SRC block. I want to go in and edit it 
after ,C-c ‘, and expect the left window to narrow in.

What happens next is the vertical split disappears, unexpectedly, and a 
horizontal split re-arranges the windows up and down.

I expect the vertical split to remain, unchanged, and for the narrowing in to 
focus on the left window in the original orientation, left and right.

I am on mac os using emacs-26.1-rc1.