I’ve seen this bug on my daughter’s (new) 4s. Even by the standards of Apple 2014 it is a good one.
All of her downloaded music disappeared. I turned on ‘Show All Music’ in Music.app settings and it reappeared — even thought it was physically on device.
iOS 8.1.1 and iTunes 12.x have some pretty impressive sync bugs (in addition to hiding hardware issues that impact sync).
It’s too late for me to avoid this iOS/iTunes pair, but I’m happy to be staying on Mavericks. I’ll look at Yosemite in the spring of 2015.
Apple has about $165 billion in cash reserves. You can buy a lot of software testing for just 1% of that. Just saying.