That's strange. Does this happen across devices? Also, could you try uninstalling the app, restarting your device and reinstalling the app? Let us know if that makes any difference.
Happens to me also and I’ve been looking everywhere for a fix. Ever since I got my new iPhone 8 a week or so ago the volume shows up. I’ve uninstalled, restarted, updated to the last version of iOS, searches through YouTube settings, etc.