Oracle 12c fast refreshing nested materialized view problems

I have a couple of nested materialized views that fast refresh. They worked fine in Oracle 11g but since upgrading to Oracle 12c (12.1.0.2.0 on Solaris) I am occasionally getting duplicate rows (the rows are not identical, usually some data has changed - I suspect the duplicates reflect the before and after change state). If I subsequently do a complete refresh the duplicate rows disappear. I have My Oracle Support access but I haven't been able to find any bugs that I can tie to this behaviour. I am aware of new out-of-place refreshes that have been added to Oracle 12c. The documentation says of the out-of-place functionality (edited for brevity). > It helps to avoid potential problems such as ... intermediate refresh > results being seen. Are "**intermediate refresh results**" what I am seeing now? (I didn't before). Is anyone aware of any relevant issues/bugs/patches?