As the discussion in the German forum showed, this is strange.
I just had a look at two of my boards:
Using 7.5V at the barrel connector closes the transistor reliably (15MOhm)
When I go down below 7 Volts the regulator is no longer able to uphold the 5 volts, the system voltage goes down to to 4.7 volts and the transistor switches through.
This is as exspected, as the comparator sees Vin =(barrel voltage -0.6)/2 against 3.3 Volts.
Having a very high load (500mA) your barrel voltage might fall below 7 volts.....
It seems there isa grey zone between the the regulator working correctly and the comparator already giving up between 6.8 and 7.1 Volts