I was looking for this too. End up digging it up from the source. You can find it in
AppleARKitLiveLinkSource.cpp - FAppleARKitLiveLinkRemoteListener::Tick
on UE5 (PacketVersion 6) its
uint8 - packet version (6)
uint32 - device id length
char - device id
uint32 - subject name length
char - subject name
uint32 - Frame
uint32 - SubFrame
uint32 - fps
uint32 - denominator
uint8 - number of blendshapes (must be 61)
float - the 61 blend shapes