Using the Direct3D11 renderer, it's possible that a call to SDL_SetRenderTarget() returns an error and discrads the command queue, thus putting the renderer in an uncertain state. When it happens, the console will display the following error : 
> __FUNCTION__, ID3D11Device1::CreateBuffer [vertex buffer]: Invalid parameter.

As I reported in Discourse (https://discourse.libsdl.org/t/bug-buffered-commands-lost-with-possible-fix-also-fixes-id3d11device1-createbuffer-vertex-buffer/26662), here's what happens :

*SDL_render.c*
- Line 1833 : SDL_SetRenderTarget().
- Line 1843 : FlushRenderCommands(), success : command queue now empty.
- Line 1900 : QueueCmdSetViewport(), one command queued.
- Line 1903 : QueueCmdSetClipRect(), two commands queued.
- Line 1908 : FlushRenderCommandsIfNotBatching()
- Line  257 : FlushRenderCommands()
- Line  218 : RunCommandQueue(), renderer->vertex_data_used equal 0 (there's no vertex data ; the only two queued commands are SetViewport and SetClipRect).
*SDL_render_d3d11.c*
- Line 2189 : D3D11_UpdateVertexBuffer(), vertsize equal 0.
- Line 1814 : ID3D11Device_CreateBuffer(), vertexBufferDesc.ByteWidth equal 0. The call fails. The function returns -1.
- Line 2190 : D3D11_RunCommandQueue() returns -1 immediately, command queue still contains SetViewport and SetClipRect.
*SDL_render.c*
- Line  228 : Command queue emptied, SetViewport and SetClipRect never executed. FlushRenderCommands() returns -1.
- Line 1908 : SDL_SetRenderTarget() returns a failure code.

Result : Since the target was valid, the renderer will render to the target, but the viewport and the clipping are invalid (SetViewport and SetCliprect command did not execute).

*PROPOSED PATCH*
Add that code at the beginning of D3D11_UpdateVertexBuffer()

  if(dataSizeInBytes == 0)
    return 0;