What is the AFR when, or just before it misses? and how much boost is it running?

The breakpoints in most 803s maps will reference the top line at around 18-19psi, so at 18-19 psi it will have enough fuel to suit the max boost it will run (around 26psi).
They will go extremely rich until the boost is increased enough to lean the mixture out once you go past 18psi.
Your miss would be around peak torque/boost on a T34.48, so it would make sense.