The UK Tax Year always runs from April 6th to the following April 5th
Were you by any chance "Contracted Out" for 90/91?
It might tell you in the State Pension Summary part of the HMRC app,
If you were contracted out for a year, you paid into a private pension for that year. You did not pay full UK Nat Ins, and cannot get a full UK State Pension of St£221.20, per week, no matter how many years you have left until pension age.
The good news is that you should have a private, defined contribution, pension (growing for 34 years) with you name on it
But you have to find it.
Also, everyone should get 3 years for free at the start. Normally the tax year of your 16th, 17th &18th Birthdays.
Each year is worth 1/35 of St£220.20 or St£6.32 per week.
So if you are more than 5 years from retirement (You need 10 years to get any forecast)
You will be able to see you got the three years, even before you pay the extra years from 2006 - present
Hope this helps