The dealer won't be able to help unless you can show them
firsthand what you are talking about. I recommend explaining that you
want the tech (preferably) or service writer to ride with you. Then
show them what the vehicle is doing. Switch places with them so they
can drive it and you can point out when it happens. Chances are, they
took it for a short drive and it did not happen, or they did not know
what to look for due to miscommunication between the servce writer and
technician.
There are several possibilities. There could be an issue with the ABS
activating unnecessarily, causing the car to take longer to stop since
ABS is dumping brake pressure when it shouldn't be. This fault can
occur due to a wheel speed sensor, and it will not necessarily cause
the ABS light to illuminate. You should familiarize yourself with the
feel of a normal ABS activation by performing hard stops on slippery
surfaces. You may hear a buzzing noise from the ABS unit operating, you
may feel some pulsing in the pedal, it all depends on the type of
vehicle. But a tech at the dealer will know what the ABS feels like if
you are not sure. Another ABS related fault is much rarer, but not
unheard of. Sometimes a dump valve in the ABS hydraulic control unit
will stick open and cause the brakes to feel less effective, and the
pedal may drop unexpectedly.
Your best bet is to work patiently with the dealer if you feel
comfortable with their abilities. Take the time to arrange a ride-along
so they can understand what they need to diagnose. Most techs do not
get paid to drive around cars trying to duplicate intermittent
concerns, so they tend to mark it as a "no problem found" after a short
road test. But they should be willing to work with you if you persist
and succeed in showing them the concern.