One of the criteria for claiming MIS is that the interest must be considered "reasonable" - this is a grey area but the interest paid may be compared with the rent limits in the area. If the interest is not considered reasonable, payment of MIS may be made for a max of 12 months at which point it ceases.
From what you've posted, it sounds as if this is why your MIS has been stopped.
You can appeal the decision, there are usually details of how to appeal included in the letter, if there isn't you can ask the CWO for details.