912ULS stoppage during stalls

On a clear sunny Groundhog and Super Bowl Day I went for the first test flight to set up SV-AP-Panel and AoA system.
Barometer was 27.81, OAT 54℉ on the ground, 6 deg C at altitude, winds calm.

Equipment:

2015 RV-12 with vented fuel tank, non-vented filler cap
ROTAX 912ULS
Dynon Skyview Touch with 2 axis-AP
SV-Knob Panel (installed 5/2019)
Gen3 HACman mixture control (installed 7/2019)
Silent Hektik VR (installed 9/2019)
SV-AP-Panel (installed 11/2019)
AoA retrofit kit (installation finished 1/28/2020 — required removal of wings and fuel tank).

Scenario:

In level flight at 6500 MSL, OAT 29℉, electric fuel pump on, HACman mixture set to full rich, I lowered engine RPM to 3,000 and began first stall to calibrate AoA. With RPM at 2500, nose up steeply and near the stall, the engine RPM fell and the engine began to run roughly. I lowered the nose right at full stall, logged the stall in Skyview (of course), lowered the nose, eased throttle forward then leveled out after full recovery. Engine recovered full RPM and smooth running, but only after 30-45 seconds of roughness with nose level.

I decided to add a second stall to the calibration. Entry to stall was similar, but of a longer duration and at full stall, RPM began to fall and in 2-3 seconds, the engine quit. By full recovery from the stall, the prop stopped. Initial attempts at using the starter failed to restart the engine. I did the usual system checks (fuel on, electric pump audibly clattering away and making good fuel pressure, all active engine monitors in the green, HACman still full rich) and began to set up an approach and landing at the paved airport less than 3 miles away. My altitude by then was over 5,000 MSL, about 3500 AGL. After several more attempts with the starter, the engine roared to life after about 3 minutes in glider mode. I re-established cruise RPM slowly and leveled off, only to have the engine quit again. This time it restarted within 10 seconds or so of running the starter.

Skyview Data logs:

• Fuel Pressure normal throughout
• Fuel Flow normal before and after stoppage
• EGT normal - i.e., no evidence of leaning
• MAP normal
• Voltage and amperage normal

Other info:

• Electric fuel pump flow tested normal after flight
• Fuel tank vent open
• No leaks from fuel tank where fuel supply line and and fuel return line connected after removal and reinstall of fuel tank 

Several hours of online searching hasn't yielded any hints regarding potential association with the use of the HACman.  Although I see references to such reports, I cannot find them.  I posted on Van's forum site.  

Further checks planned for today and tomorrow:

• weigh Bing carb floats (replaced two heavy floats 9/2019)
• inspect all fuel lines and HACman connections

I suspect the HACman only because everything else seems OK so far.  I have flown approximately 10 hours since the device was installed, and have not noticed any issues in terms of engine performance.  However, this is the first time I put the aircraft into a steep nose-up attitude at low RPM in order to perform stalls.