BitBucket: https://bitbucket.org/namida42/neolemmixplayer/issues/31/only-on-terrain-objects-only-render-on-one (For Editor issue; game issue is fixed in latest experimental code.)
Just what it says on the title, if you try using only-on-terrain objects on terrain where one-way is not enabled it is both invisible in game and in editor.
As far as the game side of things is concerned, the new rendering code for the upcoming major update fixes this issue.
I'll leave the topic open because this remains a problem for the editor.
This is now fixed in the editor. However, I won't include this fix in any released copies until a stable version of the game with this bug fixed is released.