FCPX 10.4.7 'multicam' doing weird things?

Three cameras. One audio. Highlight them all in Browser. Create 'multicam' clip. But the clips are not syncing with the audio. They are strung in a chronological line, not stacked. And on the first try FCPX automatically created 'Optimized' versions of the clips -- even though I didn't ask for them. There was already a 'Proxy' version of the clips.

Never seen this before. I've tried four or five times ... and the clips won't sync as a multicam properly. The camera audio on all three clips is solid, and high enough level, so there shouldn't be an issue there. The master audio has very good levels.

What might be happening here?