> Could that disrupt noninteractive calls?

Yes, that's why added the notice in NEWS in the "Incompatible Lisp Changes".

But, on one hand, programs should perhaps be using switch-to-(prev|next)-buffer. AFAICS, next-buffer and previous-buffer aren't even documented in the Elisp manual (they are briefly mentioned in the Emacs manual).

 And on the other, if code is using (next|prev)-buffer, silently getting the same buffer is likely a subtle bug anyway. At least now they get notice.