Help please, Tribunal "Correction"

Hello,
I have been fighting for my mothers Pip award to be increased to enhanced on both daily living and mobility since August last year. The appeal was finally heard (without our knowledge) on the 29/09/20 and was issued to both parties on the 02/11/20.
in that decision notice it Staes that the appeal has "been allowed" and my mother will be entitled to the enhanced rate of both daily living and mobility, and this will be backdated to August 2019. This was the right decision we felt.
We have waited and waited and nothing has happened. But now today we have received another decision notice from the court saying "Corrected decision notice" and it now says "The appeal is allowed" but then goes on to say she is entitled to the enhanced daily living but only the standard mobility, with a reason given that was not mentioned at all in the first letter. It says this was now issued to the parties on the 08/12/20.
I just want to know, can they do this? just change their mind? they said they had a request to change it, presumably from the DWP, but surely once a decision has been reached they cant just ask to change it?
I don't know what to do from this point on. Thankyou.

Thank you for replying.
I have rang the Tribunal service this morning and they have told me to try and get the decision "set aside", should I ask request a statement of reasons or wait until I know if it has been set aside?
Thankyou so much
Amanda