I keep saving albums and making them available offline on my Desktop, then expect to pick up my phone, leave the house and have that album saved. That's not how it works.
I wish there were an option in settings to "sync offline albums across devices"
I'm sure this isn't added by default due to hard drive size issues, but it would be a great thing to be able to toggle on and off. I recently had to listen to the same Beachhouse album 5 times on a plane because I'd saved music on my desktop and forgot to do it on my phone.