I’ve noticed this too. It says it turns on loop mode, but it doesn’t loop. I try to use this in conjunction with a sleep timer, but to no avail since it never loops.
I would really like this feature to work as my kids like to listen to stories at bed time but right now the echo is not very useful because it stops at the end of the story even if the echo tells me Plex has turned loop mode on. Come on Plex team!