The problem could be that SmartThings is looking for that plug to send a new event report of power less than 25W, or power dropped below 25W (which means it would first need to exceed 25W). Can your check the history tab for that plug and see if it is reporting any new power events during those hours? If it’s not, because it’s continuously at 0W usage, test it by running a load, first less than 25W, then above 25W if the lower load doesn’t result in the automation running.
I don’t propose this as a solution, just testing to understand why it’s not working. I’m experiencing similar frustrations with other automations that are supposed to take action when there is no motion for so long during a period of time. The problem is that rather than looking for the lack of a “motion detected” event during the period, it’s looking for a “motion stopped” event, meaning they’re must first be motion detected. This makes the automation useless if I’ve already gone to sleep before the time period.