The problem was with an Octane Volume medium loaded with an Open VDB file. Enabled, the plane is visible regardless of material options. Disabled, the plane displays as it should according to the material settings.
I was able to recreate the issue in a clean project file that is hopefully clear to see. In the first image, the volume object is disabled and the plane is invisible. In the second image, the volume object is enabled and the plane is visible. My guess is because the plane still exists in the scene even though it's completely transparent. If so, that stinks if you're trying to use opacity maps with volumes, but I think you could achieve some really neat effects using this engine quirk.
@zoppo Thanks for the tip! Changing/disabling index had no effect.
@niestudio I just did! I prefer not to troubleshoot extensively on client time, so I had to wait for the weekend.
@frankmci
zero opacity does not mean invisible
Thanks, I didn't know that. I appreciate how Octane treats materials realistically, but reality and art direction don't always like to play nicely together. It's a major bummer as someone who loves custom displacement maps AND exuberant amounts of fog.
Anyway, thanks for the help everyone.