My wife went on maternity leave earlier this year and returns before the end of the tax year in 2020. Her first pay was at the beginning of the tax year. She has been taxed as though she would continue to earn the rate she was in that specific month for the rest of the tax year. Over the tax year she will earn less than the taxable threshold so presumably she will be owed tax back at the end of the year? Does she need to do anything to claim this back or will it automatically be sent back to her in April 2020?
Any tax paid in 19/20 should be repaid after April 2020 if the payments are under the tax free allowance.
Alternatively, your wife should contact HMRC and get her tax code adjusted to reflect the changes in her circumstances. I personally would contact HMRC on 0300 200 3310.