By default, your Pixie turns on its WiFi module once every hour to sync up with the server. When the timer is sleeping, it is unaware of the schedule changes. Your Pixie timer is in the sleeping status during the time frame in your one time run. The next sync time is 2022/08/16 16:05. Therefore, it will have to wait until the next sync-up to start the watering.
If you have to create a one time run to water instantly, you can go to Settings -> Preferences-> "Keep Online" and enable the feature. Please check this article for more info:https://www.netrohome.com/en/shop/user_guides/2