All music disappears from iPhone 5 when managing manually

I have an iPhone 5 running iOS 8.2, and a MacBook Pro running Yosemite 10.10.2, iTunes version 12.1.50.

I have my iPhone configured to manage my music manually. Recently, while moving an album from my iTunes library onto my iPhone (phone physically connected to the computer), all the music stored on my iPhone disappeared. Surprised and frustrated, I began the task of moving the albums I wanted over to my phone. The music transferred fine up to a point, and then all music on my iPhone disappeared again (i.e., nothing shows up in the Music app). I also noticed that the "other" category in the memory usage bar was suddenly very large... many GBs.

Things I've tried so far:

Uninstalling and re-installing iTunes

Restoring iPhone to factory settings and then restoring apps from iCloud backup.

After doing the above, I still get the same behavior: after transferring 4-5 GB of music, it suddenly ceases to be visible to the Music app on my iPhone, and the "other" memory category is huge. The only way I've found to reduce the "other" category back to a normal size is to restore my phone to factory settings, which is frustrating.

I'd love to once more be able to move as much music as I want onto my phone without it all disappearing on me. Any thoughts would be much appreciated.