Student Loans charged interest based on previous tax year

Monkey_King

I would appreciate some feedback / financial support in regards to paying interest on a Plan2 loan based off previous tax year.

The remaining of my student loans which was a large chunk was settled in full December 2020.
I was charged an additional £500+ which I've now been told is because of the below response from SLC

"As you repaid your loan in full on 2 December 2020, the interest rate that was applied to your account was for the tax year 2019/20, as we had not yet received your end of year repayment file from HMRC, due to this being repaid throughout that tax year. This information is normally received after the tax year has ended and if a balance remains, we would then backdate and amend the interest rate accordingly.

However, when we received your end of year repayment file from HMRC on 6 August 2021, we did not backdate any interest as the interest was calculated correctly based on 2019/20 income at time of repaying in full. We cannot backdate and amend interest if a balance has been repaid in full throughout the relevant tax year. As such, the interest on your account has been applied accordingly."

For the tax year 2019/2020 my salary exceeded £40,000, in the tax year 2020/2021 my income was less than £20,000.
My understanding is SLC should backdate the interest correctly for the tax year 2020/2021, not apply interest based on my income for 2019/2020 which results in the maximum interest rate being applied for high earners. 

Effectively I'm being penalised for paying the remaining debt before the end of the tax year.

I do plan on contacting ombudsman service, communication with SLC has taken since last November to find out the reason for the extra interest. 
Also requested a link / documentation where this rule is stated

Any thoughts, does this seem right or wrong?