My son's 2015 ST does this and he's just had it serviced and he mentioned this to them but Ford said there's nothing to worry about and it's normal. Thankfully his car is still in warranty so if anything does happen it will go straight back to Ford.
It is indeed perfectly fine, I'm a Ford tech and they issued a technical service bulletin to dealers explaining that it's just a "characteristic". They do all do it so I wouldn't worry.