I didn't have that problem on my 2017 Value, but something you might try is to put it into manual mode to see if it still does it. If it does, then I would recommend checking a few things:
1. Tire Pressure (don't just believe the TPMS sensor)
2. Make sure your handbrake is fully disengaged. This may result in checking to see if your rear pads are wearing faster than the front, or that the brake is not releasing fully.
3. Shift up while in the manual mode and see if it continues to decelerate quickly.
4. Turn off the radio and AC to see if you hear a "shooshing" sound while it is decelerating. If it is, you most likely have a brake pad that is not fully disengaging.
If putting it into manual mode causes it to not decelerate as quickly, then it is the Automatic transmission trying to "assist" and is perfectly normal. Some of the Hyundai models have "braking assist", but I don't recall the Elantra having it in any trim level. Good luck!