In theory this should never happen as Plexamp should save and restore the volume. However, there must a bug causing it to reset to the default for some reason, which is 100%. If you can capture the Plexamp logs after this happens, it would be useful to see them.
We get and match lyrics in the cloud, so nothing you can do, unfortunately.
It does stay on for local lyrics, but it can’t for the premium ones as we get charged per lyric.
We’d love to hear feedback on this, as we find it quite easy to use once you learn a couple of app idioms.