I built a lower third title in motion 5 (using the FCP title preset). When I look at it in Motion over transparent, it keys properly.
I publish to FCPX, it looks right in the title browser, but when I scrub over it, the key is inverted. That is, the background of the lower third is transparent, revealing the underlying video and the rest of the screen is black. The type shows in the proper place and is modifiable.
When I drop it on my timeline, I get the same inverted key problem.
I've tried different compositing modes in FCPX with no luck.