HMRC has a number of legacy computer systems, which can result in corrupted data and incorrect tax computations. This has been an issue with class 2 NIC as we explained in our newsletter on 8 December 2016, and again on 23 March 2017.
It appears that the problems with class 2 NIC have not been addressed by HMRC, perhaps because it expected class 2 to be abolished from 6 April 2018. In fact, class 2 NIC will be retained until 6 April 2019, when it will be merged with class 4 NIC.