Reduced Personal Allowance Query

I have only 1 job as an employee, have worked for the same company for several years, having all deductions taken at source through PAYE. Checking HMRC online, my tax records are all up to date, I do not owe any tax from previous years, but for some reason when completing my self assessment (first time being asked to do so), on the summary page my personal allowance has been reduced from £12,570 to £10,390 resulting in tax owed for 2022/23 being £999.60...why would this be if my tax code has not changed and I'm not paying off a debt from previous tax year. I'm not sure why I have to submit a self assessment as an employee on PAYE, and now even more confused with a reduction and a £1k bill payable by end of Jan

Hi,

It may be that your income is over £100k and as such full allowances are not due.

Income Tax rates and Personal Allowances: Income over £100,000

Thank you.