Late payment penalty for 2021, but I have a payment plan set up?

As I was unsuccessful calling HMRC (5 days in a row call was cut off after more than 1 hour waiting time), hoping to get an answer here. I have filed my self-assessment for 2021 on-time and as I could not pay full amount, I set up payment plan. I have received two letter confirming payment plan in late March 2022, before 1st of April deadline. However I have now received a 5% penalty for paying my taxes late. According to this article this penalty should have been waived: https://www.gov.uk/government/news/hmrc-gives-self-assessment-taxpayers-more-time-to-ease-covid-19-pressures

Hi
Penalties would continue to accrue and if you pay the payment plan then the penalties may be cancelled.
Our helpline can confirm this :
Self Assessment: general enquiries
Thankyou.
Regards.