My 2012 i4 had a similar problem. Mine only happened on occasion, your seems like it happens more often.
When it was doing it, i had about 12k miles like you. Went to the dealership and they turned some kind of calibration mode on. I guess the transmission learned my old driving habits of almost all highway driving. (at that time i was entering a new job that required a lot less highway travel and more city travel, so the transmission was confused i'm guessing)
Your case may be different, but that's how I got mine to shift smoother.
Now it drives much better @ 20k miles

 

That sounds like the torque converter clutch engaging to complete the shift. It's done on most shifts. If it's learning a new driving style or the throttle is changed during a shift there can be a noticeable time spent while the torque converter clutch slowly engages dropping the engine a couple hundred RPM. In an effort to be in direct drive as much as possible and have smooth shifts, there is a lot of modulating (slipping) of the torque converter clutch and range (gear) clutches. You are not supposed to notice all that messing around with the clutches!

 

You dont want that "slush sliding" into gear. I have a heavily modified transmission in my dodge ram and it hammers into each gear. That's what you want. Being too smooth means your clutch packs to not have good friction or holding power, or your torque converter is a slush box, which is very inefficient. It would be sorta like letting the clutch out too slow as you shift a stick.

 

Thanks for the posts! She sounds normal then.