Stamp 0 being cancelled

[AB021]

Hi All,
Any help/opinion on our situation would be appreciated!
Me and my brother both stamp 4 holders applied for our father to join us here on the basis of stamp 0.
The process started in 2019, and he was eventually granted the permission in June 2022.
He applied for an entry visa within a few weeks of the receipt of that letter, however we're still awaiting a response from the visa office in relation to the status of his entry visa application. We've sent countless letters and emails to the visa office asking for an update, to no luck.
Today, we got a letter from the stamp 0 office saying that his stamp 0 offer is now cancelled, despite acknowledging our letter to them about the delay we're seeing in obtaining visa. We're absolutely shocked about this decision and can not think what can do anymore as we've already been living separated for 4-5 years, during which, my father missed the birth of his first grandchild as all our family are living in Ireland - legally of course.

Any comments would be appreciated on what can be done next!

Wish you all peace and health