If you pay Class 2 NI on a  voluntary basis as Self Employed / Business Partner, then the payment is via your Self Assessment but it must be paid by 31st January after the tax year – pay a day late and the right to make a voluntary payment is lost.

Its transpired that for 2022-23 payments, due by 31st January 2024, HMRC has made a mistake on the dates and consequently refunded some of these in error.

If you paid, or planned to pay, voluntary Class 2 for 2022-23, check your bank statements for a erroneous refund.

This article from Accounting Web explains more
HMRC refunds voluntary NIC payments by mistake

Voluntary Class 2 is normally paid because business profits are below the Small Earnings Threshold and NI isn’t being paid or credited elsewhere.  The voluntary payment protects State Pension and other benefit rights.

For our guide on the topic see:
Voluntary NI & Protecting State Pension and Benefits