This anomaly applies to IC Rev. Engineering A, build codes QFAA-AA0, QFAA-AC0, CGAA-AA0.
It is not possible to configure PPI.FORK[n].TEB register for fixed PPI channels (20-31).
Always.
Code using PPI.FORK on fixed PPI channels will not work.
A programmable PPI channel has to be used to implement the connection.