Hi Nick,
Sorry to hear you’re experiencing the same frustrations. I do believe you’re right, that it’s Mavericks (as Shane explained above). But I think I’ve also seen earlier versions of OSX sometimes only have an AVCHD file, and other times have a folder. I’m not sure what distinguishes when things happen one way or another.
I also know that you can right-click on the AVCHD file and choose “open package contents” to get at all the folders inside that you’re used to, but I don’t think there’s a way to do this from inside FCP.
– Eric