You know, If you really want to hear an opera on an Echo (I don't think the audio system of Echo is really up to the task, tho) I would just send the playlist to the Echo via BT for now until you get the issue figured out at Amazon. It's how I would handle it for now.
I am sure I could send the complete Ring cycle to Echo from my Windows PC without mixed up tracks using the Play To feature, and without using a playlist since Play To can play albums in order. The question is: Do I really want to hear The Ring on an Echo??
I am sure the same issues would occur with symphonies, too.