Does Home Office have your current address?
323A(b) that covers instances where migrant's sponsor ceases to have a sponsor licence states "
may be curtailed if" whereas
323A(a) states "is to be curtailed if", which indicates that there may be a discretion where 323A(b) is involved.
That said, I am given to understand that under such circumstances leave is indeed curtailed to 60 days within which the migrant can find a new sponsor and apply for leave to remain. One may not receive such letter immediately but it eventually arrives in 6-8 weeks.
If Home Office has your current address on record but you have not received any such letter then the best thing to do is call up Home Office and check if such a letter has been issued to you, and if so, why haven't you received it.