I also have this problem along with another serious issue: When playing a shuffled playlist to my networked speakers, Spotify unshuffles the playlist. It sometimes goes back to the beginning, but always starts playing songs in the order they are on the list. It also mangles play when queueing tracks. It happens whether I am playing from my phone (Android) or computer (Mac).
This is on a system with a GGMM E2, E5, and two E3s. It worked fine for about a year before having any of these issues.
I have the same receiver and i cannnot connect Spotify to my system via MusicCast. They are updated and firmware upgraded and on the same wifi 2,4 ghz. My system appears in the list of devices on the Spotify but i cannot connect them. I'm using a galaxy s9+ on Android Oreo.