Any time i get a list from another editor, i ask them to “commit” their multicams. Most NLE’s have a way of taking a multicam sequence, and – essentially- taking away the multicam ability.
This commitment is usually needed for the online box to do a proper conform. From Avid, an uncommitted multicam sequence will usually show proper clips from 1 of the cameras, and black from the other. I haven’t received a FCPX timeline for a while, so i can’t say for certain if it would show offline, but i would certainly start by committing your multicams, or what ever final cut calls it.