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I would suggest you to write a minimal test case.

Haven't tested this, but should be close enough


// Create a 32-bit, alpha-channeled SDL_Surface in the appropriate endian order for the given platform:
inline SDL_Surface * create_surface(const int width, const int height)
{
	/* SDL interprets each pixel as a 32-bit number, so our masks must depend
	   on the endianness (byte order) of the machine */
	#if SDL_BYTEORDER == SDL_BIG_ENDIAN
		return SDL_CreateRGBSurface(0, width, height, 32, 0xff000000, 0x00ff0000, 0x0000ff00, 0x000000ff);
	#else
		return SDL_CreateRGBSurface(0, width, height, 32, 0x000000ff, 0x0000ff00, 0x00ff0000, 0xff000000);
	#endif
}



std::vector <SDL_Texture *> frames;



int add_frames_from_tile(const char *image_filename, const unsigned int number_of_frames, const unsigned int frame_width)
{
	SDL_Surface *tiles_surface = IMG_Load(image_filename);

	const int total_width = static_cast<int>(number_of_frames * frame_width);

    // Create a frame surface for copying the individual frames to:
	SDL_Surface *frame_surface = create_surface(frame_width, tiles_surface->h);
        SDL_Texture *frame;

	SDL_SetSurfaceBlendMode(tiles_surface, SDL_BLENDMODE_NONE);
	SDL_SetSurfaceBlendMode(frame_surface, SDL_BLENDMODE_NONE);
	
	SDL_Rect source_rectangle = {0, 0, static_cast<int>(frame_width), tiles_surface->h};

	for (; source_rectangle.x != total_width; source_rectangle.x += frame_width)
	{
		// Sprite tiles will be overlaid without this line:
// SDL_FillRect(frame_surface, NULL, 0x000000);
                SDL_BlitSurface(tiles_surface, &source_rectangle, frame_surface, NULL);
                frame = SDL_CreateTextureFromSurface(Renderer, frame_surface);
		frames.push_back(frame);
	}
	
	SDL_FreeSurface(tiles_surface);
	SDL_FreeSurface(frame_surface);
	return 0;
}

Created attachment 2484 [details]
test case

Created attachment 2485 [details]
input image for the test case

Here's a small test case, just loading and saving surface/image.

This issue happens because the *alpha* channel of a palette surface, is in fact a color key.

So you don't have to set the "blend mode" to none, but disable the color key, then re-enable it on the output.

Since color key is color, you need to retrieve it, convert it from the input pixel_format, to the output pixel_format, to set it back. ( 


Maybe SDL_SetSurfaceBlendMode could be translated as "disable colorkey" for palette surface ? but to re-activate it, you need the explicit value.