This is very strange. Are you sure they have been imported ... in other words they list in the smart playlist "Last Imported".
If they are listing there, you can add them to any playlist from there (probably as you are already doing).
But good you have worked out a way around this.

As suggested, you could backup, delete the app, download, import backup.
We have had some problems with certain devices, models such as Nexus, where, if importing a playlist from Drive twice in a row might cause problems. This is solved by force quitting the app or sometimes swiping it to close the task in the Recent apps menu.

You could use the Contact us in More>Help and we can look into this further. We would need some details of your device and what you are exactly doing so we can look into this.
Thank you.