We never got to the bottom of it, but I felt like the issue with those was specifically that they did have to be signed for.
The postman would take the general post into reception and then the normal ones would just get processed. The signed-for ones would get signed for by security. They probably then kept them at some kind of security desk, “because they’d been signed for, and needed to be secure”. Monzo staff were probably supposed to go down and get them, but due to a general lack of consistent process they sometimes went missing at the building security desk. I can easily imagine somebody putting it in a locked drawer or cabinet, then their shift ends and they go home. A different person is there later and when someone from Monzo comes down to check the post, they don’t know about the cheque in the drawer.
I always had something like that in my head anyway, no idea if it’s true!