This sounds like the same issue I'm having. I am totally new to FCPX though so it could be something I'm doing.
The issue I had was -
Any clip on the timeline will show and play back OK in the viewer
However when I then choose a clip in the browser it intermittently (99 times of 100) wont show as being selected in the viewer and also wont play back. The viewer just shows the project name on my time line. However if I look at inspector it does show that I have selected the clip in the browser. In addition I can play the clip in the browser & hear it. However as I say it doesn't show in the viewer.
That said, when I closed the app and reopened all now seems to be working OK. I do have two monitors.
I wish I'd know that 3 hours ago.
Did you have any success getting your issue fixed?
I'm thinking this must be a bug in FCPX
Best wishes getting it fixed