Could some one with an unmodified M&P 40c try this.

With your pistol unloaded and pointed in a safe direction and the manual safety off pull back on the trigger until you feel contact with the sear.

Now put the safety back on while still holding the trigger back.

Now continue pressing trigger, mine goes click with the apex parts. Please let me know how a
unmodified M&P works under the same conditions.

In a stressed situation using the apex hard sear, this could be a real safety issue.

Oh heck, this is even worse, just pulled back trigger, set safety, let go of trigger and then pulled trigger. Click.

Further playing with it shows that the safety goes into a half position which sounds and feels good but it isn't good.

Solution for now:
Don't have your finger on trigger when putting thumb safety in safe position.

Szumi