>>> And here is a micro-patch to fix that behavior.
>>> 
>>> +  ;; Terminate the search if point has moved to another buffer.
>>> +  (unless (eq isearch-buffer (current-buffer))
>>> +    (isearch-cancel))
>> 
>> Have you tested this with 'multi-isearch-buffers'?
>
> No, because I did not know that multi-isearch-buffers exists.  I see 
> that it uses multi-isearch-current-buffer in a similar way than 
> isearch-buffer, but currently I don't know anything else about 
> multi-isearch-buffers, so I cannot try to fix this.
>

Okay, I looked a bit closer at multi-isearch, and it seems that the 
attached patch works with mutli-isearch, too.