Unless I missed something the AAIB report does not tell that a failure occurred...
Yes you missed something....other posters have quoted the AAIB several times.
The AAIB states it is continuing to investigate the apparent loss of tail rotor control.
They do not categorically state there WAS a Tail Rotor Control failure that caused the accident.
Combine the AAIB statement with the Video's that are available and one can easily assume a Tail Rotor malfunction of some sort occurred.
Did the AAIB not state in its Report that the aircraft was responding correctly to Yaw Inputs by the Pilot until the onset of the yawing?
Then add in to your consideration of the AD:s that were issued re the Tail Rotor Control Assembly and ask yourself why that happened very shortly after the accident.
Yes....I would say you have missed something.