Normally, I would say it is unlikely that it is the MAP sensor (but, not impossible). MAP sensors, when they fail, normally tend to a fixed output voltage or no output voltage. It would take some kind of weird planetary alignment where the MAP sensor works fine from 0 - 100 kPa and then goes bad when you hit 135 kPa and then sticks there. Also, on an OEM ECU if the MAP sensor goes out of range the ECU will generate an error code. However, the uncertainty is that the AEM FI/C has its own MAP sensor and clamps the voltage signal to the OEM MAP sensor to prevent the signal voltage from going too high when in boost and generating an error code. It might be possible that something has happened to the AEM FI/C; but, I think lower probability.
I am thinking that perhaps you have a fuel pressure regulator problem. As boost increases, the MAP referenced regulator raises the system fuel pressure. If the FPR is sticking or failing to regulate the pressure properly at high boost you may be running high fuel pressure resulting in low AFRs. It should be relatively easy to check. Assuming you are running the same fuel pressures as OEM, do the FPR test as set out in the service manual. If that confirms that fuel pressure under idle is normal, connect the FPR reference line up to one of those Mity Vac style hand held pumps that does both vacuum and pressure. Using the pressure function raise the pressure on the reference line up to your maximum boost level and monitor the fuel pressure while you are doing this. With a normal 1:1 FPR, you should see the fuel pressure rise smoothly 1 psi for each 1 psi that you increase the pressure on the reference line. Gradually release pressure on the reference line and watch for fuel pressure to drop at the same rate as you are releasing pressure on the reference line. If the FPR tests out normal, then time to look someplace else.
The OEM ECU uses the O2 sensors to adjust for fuel mix errors. The O2 sensor feedback signal can correct for a limited amount of error from the fuel pressure regulator. However, the O2 correction has limits and if the fuel pressure gets seriously out of range, it may not be able to control the AFR. That might be why the problem only shows under boost. As an observation, if the AFR goes seriously out of range (10 would be seriously out of range on an OEM ECU) and stays out of range, the OEM ECU would normally be generating an error code. Is your ECU generating any error codes?
As a matter of interest, if you measured an AFR of 10, you must have a wideband O2 sensor. Does the system have an additional bung for the wideband for monitoring only and retains the OEM narrow bands for the OEM ECU? If you are using the wideband for O2 control, do you know that the wideband controller or sensor hasn't checked out, particularly if it is one of the older LSU 4.2 sensors which don't have the durability of the newer LSU 4.9 sensor.