Suppose that your United Kingdom tax code changes mid way through the tax year e.g. your spouse successfully applies for the marriage allowance, which increases your personal allowance. Does this mean that your personal allowance is updated so that, for the current tax year, your 'old' personal allowance is completely irrelevant? Or, is your personal allowance for the year some kind of mixture of the old and new personal allowances?
When you receive a new PAYE coding notice it supersedes any previous notices you have received for the same tax year.
The new coding notice will incorporate the changes from when they took effect. In the specific case of marriage allowance it applies to the whole year as your partner is effectively transferring some of their personal allowance for that year to you.