SDL_RenderPresent memory leak?

I’m running SDL2 in Windows 10 using the OpenGL backend. If I enable the ‘Shared GPU memory’ column in Task Manager it reports what appears to be a huge GPU memory leak (several megabytes per second) which only stops if I comment out the call to SDL_RenderPresent() in my code. Exactly the same thing happens if I replace it with SDL_GL_SwapWindow().

Except that I don’t believe it! I can run the program for several hours and it doesn’t fail, despite Task Manager reporting an impossible amount of memory usage (the PC concerned has a total of 8 Gbytes CPU + GPU memory). So why is it lying?