There has been a big refactoring of the PBR shader code to split it into functions to be able to implement the NME PBR blocks as well as to ease future PBR improvements (and also for the sanity of the developpers who had to deal with a big chunk of linear code previously, as everything was held in pbr.fragment.fx).
All the custom defines are still there, but possibly in one of those news functions, meaning some variables may not be available to you at this point: you only have access to the variables defined inside the function or to the parameters passed to the function.
This can be a breaking change depending on your use of the custom defines, but unfortunately the pbr shader code (and all the internal shader code in Babylon) is not guaranteed to stay backward compatible as it will evolve according to the needs. Of course, we try as much as possible to avoid such breaking changes whenever possible, but sometimes it is not possible.
Hopefully, the PBR support in the NME will help to get around some of these problems!