This is a very niche issue, but I expect this to be an easy fix for Spotify's programmers.
When using Spotify Connect, I'm able to control the volume of my Android TV with my Android phone, just like I can through the "Android TV Remote" app. That's awesome! Here's the problem, though: when adjusting the volume, a slider appears on the screen.
My Android TV is set up to adjust my receiver's volume, and my receiver doesn't use a slider. Thus, when I try to adjust the volume, I'm only able to turn it up or down a limited amount. e.g. If my receiver was on volume 50% when I opened Spotify Connect, I could only adjust the volume between 40% and 60%.
By contrast, no slider appears in the "Android TV Remote" app; I'm able to turn the volume all the way up or down, whether or not Android TV is set up to adjust the receiver volume.
My idea, then: Make Spotify Connect's volume control more like that of the "Android TV Remote" app!
Updated on 2019-04-30
Hello and thanks for the feedback!
This sounds more like an issue that's worth investigating.
Could you please search for that issue in the respective help section and provide more info about the issue. If it doesn't exist you can create a new topic and let us know about it! Then we can check it out there.