In the past I've always done this by building the return path with a continuously banked track. This works with everything, coasters, trains, car rides etc...
Like this, essentially the same solution saxman seems to have discovered.
I was just using the launcher and updated before opening the game. It was working fine until this point.
Also, what does RCT1-Import mean?
EDIT: Downloaded the latest development build, still having the same issue.
Using latest build b81c3ad, files save simply as "file" and not as SV6. They do not show up in the file browser in game, but can be loaded via callup in the file name bar.