On my iPad simulator (Xcode 9.4) and iPad hardware, I noticed that SDL_GetCurrentDisplayMode(0, &current) reported these screen width and height values before SDL_CreateWindow is called:

current.w=320
current.h=480

And these after:

current.w=768
current.h=1024

These numbers did not change on iPhone simulator or hardware, nor did they change on Windows, Linux, Mac or Android.

I tracked the change down to UIKit_GetDisplayModes.  Here's the call stack:

SDL_CreateWindow(const char *title, int x, int y, int w, int h, Uint32 flags)
  SDL_FinishWindowCreation(window, flags);
    SDL_SetWindowFullscreen(window, flags); - w and h get zeroed out here
    SDL_ShowWindow(window);
      SDL_SendWindowEvent(window, SDL_WINDOWEVENT_SHOWN, 0, 0);
        SDL_OnWindowShown(window);
          SDL_OnWindowRestored(window);
            SDL_UpdateFullscreenMode(window, SDL_TRUE);
              SDL_GetWindowDisplayMode(other, &fullscreen_mode)
                SDL_GetClosestDisplayModeForDisplay(SDL_GetDisplayForWindow(window),
                                             &fullscreen_mode,
                                             &fullscreen_mode)
                  SDL_GetNumDisplayModesForDisplay(display)
                     _this->GetDisplayModes(_this, display)
                       UIKit_GetDisplayModes(_THIS, SDL_VideoDisplay * display)

In my program, I simply made sure I only called SDL_GetCurrentDisplayMode once to get consistent behavior across all platforms.  Still, it was really confusing and I spent a lot of time messing with it so I figured I should log a bug.