I had this happen to me--at the Cape Town Marriott Melrose Arch--while I was on a stay there last year. My reservation disappeared and was replaced by a reservation for a completely separate person (different name, member number, and reservation number). Since I was actually at the hotel, I confirmed with the front desk that my original reservation still showed up and was untouched. Points and stay credit ultimately posted appropriately, and the other person's reservation just vanished after their checkout.

Definitely a tech issue, since even the other person's membership number was different. Maybe a tech issue at that particular Marriott (OP doesn't specify which Marriott in Cape Town...)