A basic tax code should be 1100 (because your personal allowance of tax-free pay is £11,000 and your tax code is obtained by dividing that figure by 10). However you've taken up the option to transfer 10% of your personal allowance to your partner (so that your own allowance drops to £9,900), resulting in a tax code of 990. (The N on the end is an indicator that you've given away 10% of your allowance).
So your 'old' tax code would seem to be in order. Your new tax code however appears to be based upon a revised personal allowance of just £4540, which does indeed seem to be wrong. The good news though is that the T on the end indicates that the allowance is subject to review by HMRC, so there's every chance that the problem might correct itself in the next period.
In the meantime though it would still seem best to ask how your new tax code has been arrived at.