Quarter of pension suddenly lost to "tax" from March 2022

After receiving the SMS below, my mother in law lost a quarter of her pension to "tax". Surely this cannot be correct? Which pensioner can afford to loose such a huge portion of their already strained budget? Something is wrong here, Sentinel please respond with answers. SMS:"Dear Pensioner, SARS instructed Sentinel to apply a SARS determined tax rate with effect from March 2022. This will impact the monthly net pension paid to you. This SARS tax % is reflected on your March 2022 payment advice, which can be found on the Sentinel online member portal."