There was a similar post to this yesterday where an item was dropped off and entered into the RM system but them immediately showed signed and delivered.
It could be an issue with Royal Mail.
I would advise ignoring this 'quirk' for now and waiting to see if your item is correctly delivered.
You can open an item not received case one day after the latest estimated delivery date.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)