This is interesting, because "officially" the Flat Face trigger requires the Forward Set Sear, and isn't supposed to work with the Duty/Carry sear/kit.
"***This trigger will only work in a Smith & Wesson M&P pistol which has an Apex Forward Set Sear installed. The Apex sear may be identified by the 3 dots in the surface on the right side of the sear.***."
Now, that may just be Apex being a bit cautious b/c the very short travel and reset length of the Flat trigger is designed around having the FSS sear/kit installed, while the Duty/Carry kit is a different geometry all together. They may define "requires" as we want you to buy the FSS/kit b/c the DC kit may not be 100% reliable with the Flat Face trigger - that I don't know. But it is interesting that you got it working with the Duty/Carry sear without issue...

. Keep us posted...