Rang and learned some things about how they allocate certain years.
Perhaps this has been covered already, but if not...
If you pay from a UK account, you are supposed to send separate payments for Class 2 and Class 3 amounts, using different reference numbers specific to those classes.
If you pay from an Irish (overseas) account, you can pay all in one payment, with one overseas reference number. The system is set up to automatically allocate to 1) the last six years, and 2) any Class 3 amounts owed. This may explain why people were seeing the oldest and most recent years only being allocated. My wife and I both had our 2006-2007 counted as partially Class 3.
They will figure out eventually the rest is for any other Class 2 payments outside of the last six years. But it would help to write to them or call them.
The rep put a note in on our records that we intend to pay 2006-2025 inclusive, including Class 2 and 3, so that when a human reviews it they'll know where it all goes.
Also, the "bill" we received in a separate letter shouldn't have been sent. The amount reflects 2019-2024, plus any Class 3. But we were told to ignore it and use the statement letter with annual amounts we can pay as the true numbers.