Thanks for reaching out to the Community, @SharkyBoi!
I have always experienced this as well, so unfortunately, I believe it is the standard setting for Spotify rather than an individual issue that you alone are experiencing or that can be resolved. I have found that the best way to avoid the volume change on my phone is to close the Spotify app on my iPhone when I play on my laptop.
This is a good suggestion yet it does not fix my problem.
Even I closed the app in my phone, spotify still recognize my phone is around and whenever I pick it up I can see the song that I am playing and in max volume. Saw this on my lock screen, so problem still exist.
I experience the same issue when casting to my Android tv (shield TV). I have found it is happening because my android tv device sets its volume to max because I have a receiver plugged in and control volume with it (so the android tv has a fixed, set volume that you don’t adjust). Spotify just sees the android TVs volume level (max) and for some reason sets my phones volume to this same level to match it. I’m assuming this is to allow me to adjust the volume of the android tv when it’s playing music by using my phones volume controls.
This happens on both my iPhone as well as android phone.
I can turn off fixed volume on my shield and then the volume control works like I’m assuming Spotify intends. It’s still dumb that me adjusting the volume level of Spotify is changing my phones sound output tho. After using Spotify to cast and I’m finished listening to music, my phone is left in a max volume state.
I consider this a significant bug that Spotify needs to fix.
Having a similar issue. When I'm listening on my PC and Spotify on my phone connects (which happens spontaneously), it will change the volume in Spotify on my PC to match whatever my phone's media volume is, which is usually very low. Very annoying.