This is a bug introduced in 10.0.8 and applies to both 5D2 and 5D3 imports (i have not tried it with any other camera).
As FCPX ingests 5D3 files with timecode I don’t want to lose that facility. Dragging and dropping the .mov files gets over the problem but I lose the timecode info.
Canon, please sort this out. I never had any problem ingesting files until this update.
Moira