I had the same problem (iphone 5). It put a bunch of my contacts under "F" and "false false" next to these names. I went to the apple store last night. They had me change "sync contacts with icloud" instead of yahoo. I thought that would help-but it didn't. I edited some of the contacts last night before I went to bed. I erased the "false false", and then the contacts went back to their correct letter in contacts. This morning when I got up, they were all back in F. Tried re-entering them several times, while I was on icloud.com on my PC, but they kept going back to "F". Starting to get very ****** off, so I just deleted my Yahoo mail account from my iphone. Fixed!!!
I have had contacts erased from my iphone on several occasions, so it is getting very aggravating for me. When it comes time for a new phone, this contact issue may be a dealer-breaker, for me, for getting a new iphone. After all, the main use for my iphone for me, is to call people. If I keep losing their phone numbers, it's not worth it for me.