If it is under warranty, it goes back to the factory, PERIOD!!!
Trying to fix it yourself, or possibly even a gunsmith doing it can violate the warranty. (gun has been "tampered with")
If you ARE going to tinker, and you think its the sear spring, replace the spring (and do NOTHING ELSE) and see if the problem is cured, or not.
Hammer "following" the slide is usually a matter of sear/hammer engagement. Not only do the angles have to be right, but the amount of engagement also matters, a lot.
For what ever reason, the "jarring" of the slide shutting is bouncing the hammer out of full cock engagement with the sear, and the hammer is falling, being caught by the half cock notch.
Call the maker, describe the problem, get their instructions on shipping, and send them back the gun. (and if the gun has had "unauthorized" work done on it, be prepared to pay for the cost of repair.