I'm not sure what could be the problem. The IPR and the final rendering to disk are the same, unless there was a problem with the motion blur while rendering sequences or something like that. Have you tried to render the scene in "Full Scene Reload" mode?
In any case, if you want to send me the scene, I could take a look at it here.
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini