To factory reset the TRADFRI control outlet, press and hold the reset button (pinhole underneath the light, located at the top (bottom if UK) of the outlet) with a paperclip until the white light starts fading. Hold onto the button for a few more seconds, then release. After this, the outlet will automatically connect.
Perhaps similar issue Ikea E1703 smart plugs keep disappearing and can't be re-paired reliably · Issue #6262 · dresden-elektronik/deconz-rest-plugin · GitHub