Yes, I have noticed the same. There are two different issues.

1. Contact data just randomly disapears. However, not fully - leaving name OR number.
2. If you add number to the FB or Google contact and there is some sort of issue with FB or Google log in, then the result is as follows - number is stored without name and not linked anymore to the relevant Google or FB contact. Actually Nokia N9 had exacly the same issue.