Drone brood in the last week of September — drone-laying queen, or still normal in North Carolina?

North Carolina, third year, four hives. Last full look yesterday before I mostly stop opening things, and one hive threw me. Three of my colonies kicked their drones out weeks ago, the usual sad parade on the landing board. The fourth, headed by a queen from a July supersedure (I marked her, white dot), still has a solid patch of capped drone brood — a palm-sized area along the bottom of two frames, plus maybe fifteen scattered bullet-shaped cappings up in the worker comb. The worker brood pattern around it looks good, tight and even, and I saw her, and she's laying. But my brain keeps going "drone brood this late = drone-laying queen = she's running out of sperm" and now I'm wondering if I should be finding a queen somewhere before it's too late. Is late drone brood normal this far south, or is this an early warning? What would you look at to tell the difference?