Why has my tax code changed from 989L to 1257L?

Recently my tax code changed to 1257L which means I am earning more per month after tax.

Looking at the 989L tax code I was on previously, it seems this is reserved for those who have had their tax free allowance reduced due to untaxable income.

However as far as I am aware, this does not apply to me. I’m fully employed and earn 70K with no additional sources of income and my circumstances haven’t changed for a number of years.

Does this mean I have been on the wrong tax code and should be entitled to a refund?