I’ve created an account just to thank you for this. So 3 years after you’ve reported the issue it still exists! I’m using GH4, FCPX 10.2.3 and OSX 10.11.6.
I was pulling my hear our trying to figure out what was causing this issue. I never would have suspected the SD card. Actually it’s more likely that it’s a filesystem problem. The filesystem of the SD card is exFAT while your SDD/HDD will have likely be HFS+.
It’s annoying that you first have to copy the files before you can import them into FCPX but I’m happy that this workaround exists!