UnityPluginLoad not called in standalone

I have a native plugin dll, that defines several functions, and contains UnityPluginLoad. When working in the editor UnityPluginLoad is called when Unity loads the dll, as expected, and all my other native functions work. In a standalone build UnityPluginLoad never appears to be called by Unity, but my other native functions work fine. So this does not appear to be a dll loading issue. Any idea whats going on here? Unity BETA 5.5.0b6 (I require the new GetNativeVertexBufferPtr api) Windows x64 build