2019–2023 RAV4 Hybrid: Why won't the engine shut off at stops when it's cold out?

When the weather was warm, my RAV4 Hybrid would go fully quiet at red lights. Now that temperatures have dropped, the gas engine stays on nearly all the time, even when I'm parked with the car in READY mode. Is this expected behavior, and what is the engine actually doing? Does running the heater or defroster make it run more often, and are there ways to reduce how much it runs?