Oh dear it sounds like a software roll back to me. Something has gone wrong with their system and its put you back to a previous state on the records.
It happens and I would expect it has affected a fair number of people on their system. I wouldnt worry too much about data protection, your data has not been shared with anyone else or made public. But your correspondence was sent to a previous address, so demand that explanation you have requested and a full apology. Its a clerical/electronic error that is probably causing a few people to scratch their heads.