Just encountered this little bug in FCP7 where I was creating a 4 way split screen and added some black bars to the horizontal and vertical crossing points to neaten up the edges. When un-rendered all the layers appeared and worked perfectly. When I came to render or export the horizontal line disappeared. No matter what I tried (trashing the preferences, switching the layer hierarchy, creating the black bars with colour matte, then slugs etc) nothing worked. After a bit of googling I discovered someone else having a similar problem which they solved by resetting the crop filter. Which obviously I cannot do as I need the crop filter to create the lines…
So I just made some new lines in Motion and imported them and now it all works! Hopefully if you’re reading this, I can save you some wasted time!
– Edit: I now realise where the error had occurred. In the crop filter you can set the bottom to 51 and the top to 50 and a black line (or whatever colour solid you have) will appear when un-rendered, but of course at those settings it doesn’t really exist. Change the setting to 49 and 50 and the problem is solved.