What causes this deformation in sdl2.sdlgfx.circleColor?

Question

I'm writing a geometry drawing tool in pySDL2 with as little reliance on other libraries as possible. I'm trying to use the built-in primitive drawing functions to achieve this, but I'm running into strange behavior when the input values grow to a certain size. For circleColor(), at least, it looks like a radius of 1024 seems to be the point where things get weird.

For example,

import sys
import sdl2
import sdl2.sdlgfx
import sdl2.ext


def run():

    sdl2.ext.init()
    window = sdl2.ext.Window("circle test", size=(1700, 900))
    window.show()

    if "-hardware" in sys.argv:
        renderflags = sdl2.render.SDL_RENDERER_ACCELERATED | sdl2.render.SDL_RENDERER_PRESENTVSYNC
    else:
        renderflags = sdl2.render.SDL_RENDERER_SOFTWARE
    context = sdl2.ext.Renderer(window, flags=renderflags)

    color = 0xFFFFFFFF
    x, y = -400, 450
    r = 1023
    sdl2.sdlgfx.circleColor(context.sdlrenderer, x, y, r, color)
    x, y = 0, 450
    r = 1024
    sdl2.sdlgfx.circleColor(context.sdlrenderer, x, y, r, color)
    
    context.present()
    running = True
    while running:
        events = sdl2.ext.get_events()
        for event in events:
            if event.type == sdl2.SDL_QUIT:
                running = False
                break
    sdl2.ext.quit()
    return 0


if __name__ == "__main__":
    sys.exit(run())

deformation

Anyone know what could be the cause?