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It seems that the software renderer can't properly render textures with 8-bit alpha channels. Here is a simple example.

This may or may not be a duplicate of bug #1550 (which is about surfaces, not textures) or similar, depending on the internal implementation of texture rendering in the software renderer.

#define SDL_MAIN_HANDLED
#include <SDL.h>

int main()
{
  const int Width = 400, Height = 300;
  SDL_Init(SDL_INIT_EVENTS|SDL_INIT_VIDEO);
  SDL_Window *pWindow = SDL_CreateWindow("Window", 300, 300, Width, Height, 0);
  SDL_Renderer *pRenderer = SDL_CreateRenderer(pWindow, -1, SDL_RENDERER_SOFTWARE);
  SDL_Surface *troll = SDL_LoadBMP("pathTo/SDL/test/shapes/trollface_32alpha.bmp");
  SDL_Texture *texture = SDL_CreateTextureFromSurface(pRenderer, troll);
  while(true)
  {
    SDL_Event e;
    SDL_WaitEvent(&e);
    if(e.type == SDL_WINDOWEVENT && e.window.event == SDL_WINDOWEVENT_EXPOSED)
    {
      SDL_SetRenderDrawColor(pRenderer, 255, 255, 255, 255);
      SDL_RenderFillRect(pRenderer, NULL);
      SDL_Rect rect = { 10, 10, troll->w, troll->h };
      SDL_RenderCopy(pRenderer, texture, NULL, &rect);
      SDL_RenderPresent(pRenderer);
    }
    else if(e.type == SDL_QUIT) break;
  }

  SDL_DestroyRenderer(pRenderer);
  SDL_DestroyWindow(pWindow);
  SDL_Quit();
  return 0;
}