When we run this code:
import pygame
w=2560
h=1440
print(w*h)
screen = pygame.display.set_mode((w, h))
pygame.draw.ellipse(screen, pygame.Color(255,255,255), screen.get_rect())
clock = pygame.time.Clock()
pygame.display.update()
clock.tick(.25)
we get this "ellipse":
It seems to be a function of total area of the ellipse or something? As we shrink it down, it starts to work correctly again - but if we increase the other dimension (say, shrink height and increase width), it eventually breaks again. It looks like if it's more than about 3.4M pixels it breaks or something, but I haven't narrowed it down, and not totally sure it's area that's related to the problem.