Hello @Scotia 

Apologize for the delayed reply, but I'd like to some information may help explain  what happened:

If a plug turn off itself at a certain time, most likely it is affected by some settings, you can check below settings 

• Please try to remove Schedule / Away Mode/ Timer/ Smart Action/ Scene settings on smart device  in case those settings affects the bulb:  https://www.tp-link.com/support/faq/2815/
• If it works with any third party smart app , like Alexa /Google /Home automation etc., please unbind or remove settings temporarily while test 
• Confirm device firmware and app are both up to date. 
• Factory reset kasa device and reinstall it on Kasa app and see if issue is gone

How to reset TP-Link Kasa smart switch and plug?

Hope this helps.