Unity 5.6 iOS GVR audio spatializer incompatible?

I am using the GoogleVR plugin for the GVR Audio spatializer plugin, which works fine in the editor and on Android, but when building for iOS the sound appears to be regular stereo. Furthermore, the logs show the following warning a dozen times:

Audio source failed to initialize audio spatializer. An audio spatializer is specified in the audio project settings, but the associated plugin was not found or initialized properly. Please make sure that the selected spatializer is compatible with the target.

This would normally suggest that the spatializer plugin isn't chosen in the audio settings, yet it is set up correctly.

I am using Unity 5.6.0f3 on Mac OS and using the GVR sdk version 1.40 with iOS as the target platform.