It's a known issue. I have a 2016 ST1. The idle will dip down to about 500 RPM when you come up to a light and act like it's going to stall but it doesn't.
It has nothing to do with the clutch, although that's what I initially thought because disengaging the clutch without applying gas will certainly stall out the car. I started putting my car into neutral as I rolled up to a light and the same thing occurred, so I realized it wasn't the clutch.
Someone else here posted the below TSB and it worked for me so decided to pay it forward.
Below are the details for the TSB written against it.
2015-2016 Focus ST- 2.0L GTDI-May Experience Idle Speed Dips - Possible MIL AndDTCs P2610 And/or P08B9
Some 2015-2016 Focus ST vehicles equipped with 2.0L Gasoline Turbocharged Direct Injection (GTDI) engine may exhibit an engine speed dip with or without an illuminated malfunction indicator lamp (MIL) and/or diagnostic trouble codes (DTCs)P2610 and/or P08B9 stored in powertrain control module (PCM) memory. Reprogram the PCM to the latest calibration using IDS release 99.02 or higher. Make sure you are connected to the internet when entering module programming to obtain the latest updates. Calibration files may also be obtained at www.motorcraftservice.com
See TSB for complete details.
I had to take mine to the dealer and have them reproduce the behavior, took them about half the day. They flashed the PCM and have not had the issue since. Don't expect them to flash the PCM, just because you "want them to". So you might have to leave the car with them awhile. Try to explain the exact details when it occurs most often so they can attempt to reproduce.