I own a number of Echo products, including 2 second generation Echos, and notice the following issue ONLY on my second generation Echos.
When I'm listening to Spotify on my second gen Echos and say, "Alexa pause," or "Alexa stop," the music initially lowers while the Echo listens to my request, but then gets louder for a second or two, again, before it stops. Performing the same workflow on my other (non-second gen) Echo devices produces the desired result...the music lowers to listen to the request and then stops, immediately, without getting louder again. Additionally, I have noticed this only happens with Spotify...while using Amazon Music on my second-gen Echos, the sound stops right away, after lowering and hearing the request.
All of my devices have the most-current software version installed and are in excellent wi-fi range.