Sigh, I have now been trying to sync music using all the hints and tricks since I received my iPhone 6 on the 19th and am now really fed up. Countless restores, changes of settings, cable syncs, WLAN syncs, iTunes Match downloads - nothing helped. After a second or third sync (backups) the music always disappeared at last while trying to play it.
Such an issue may not happen with an Apple product as they are in the highest price segment and people usually buy their products BECAUSE they don't want to have any issues with the most basic features.
I have now requested a full return to get my (converted) USD 1140 back - this is the price of a 128GB iPhone 6 in Germany (which was meant to hold all my music).
Thanks for all your help and good luck for those who could not fix the issue as well - maybe Apple will take care, but after reading some comments that this is a known problem at least since iOS 7, I doubt it. Luckily it has just started now for me while I am still able to return the phone.
Kic