Version 10.4 issue importing/transcoding AVCHD files

Running under the newest version of High Sierra, my iMac was recently upgraded to Final Cut 10.4. When importing AVCHD video files or "optimizing" them to ProRez, the resulting transcoded files are missing frames...they won't playback smoothly and, of course, won't render properly.

This was not an issue with Final Cut 10.3.4. or, in 10.4, if the AVCHD file was converted to ProRes with different software. Also, a library created under 10.3.4 and upgraded to 10.4 doesn't exhibit the dripped frames.

To deal with this "bug", I've gone back to 10.3.4. Anyone else experiencing this problem?

And what's the most effective process of reporting this to the FCPX team at Apple?

Many thanks,
Bob

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