To fix this issue, you can try the following:
Disconnect the devices: If you are using the same Spotify account on both your desktop and phone, try logging out of the app on one of the devices and see if that fixes the issue.
Check your settings: Make sure that the “Sync volume across devices” setting is turned off. To check this setting, go to the “Settings” tab in the Spotify app and look for the “Playback” section.
Restart the app: Sometimes, simply closing and reopening the app can fix minor issues like volume synchronization.
Clear the cache and data of the app: This can help resolve any issues with the app’s data that may be causing the volume synchronization issue. To do this, go to your device’s settings, select “Apps” or “Applications,” find the Spotify app, and tap on “Clear cache” and “Clear data.”
If you have tried these suggestions and are still experiencing the issue, you can try reaching out to Spotify customer support for further assistance. They will be able to help you troubleshoot the issue and get the volume working properly on both devices.