Until Apple issues an update to correct the problem, one temporary workaround that users have had the most success with is to open the Watch app on iPhone and toggle the 24-Hour Time switch in the Clock app settings, wait a few seconds, then toggle it back to its original position. The paired Watch should update, and the affected complications should display weather data again (until they don't again – your mileage may vary).
source: https://www.macrumors.com/2023/09/26/watchos-10-weather-complication-issues/
