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When closing and relaunching the app on a multi-monitor system, the Spotify window retains its X/Y coordinates, but not which monitor it was on. So, for example, I keep the app in the top-right corner of my second monitor; on relaunch, it appears in the same position but on the main monitor.
This is using macOS 10.13.1 and Spotify 220.127.116.112.g19436fa3 on a Late-2014 5K iMac.