The problem is being caused by trying to forward your PBX inbound calls to the Google Voice number. That is backwards from the design of Google Voice, which is intended to be the single point of contact number, which then forwards calls to linked destination numbers and/or OBis and to Hangouts.
The fact that it used to work is interesting, but irrelevant. Google makes changes to their infrastructure on a regular, ongoing basis, and there is no guarantee that an unsupported configuration will continue to work. In addition, PBX software changes all the time, as do the configurations users make to their PBXs that may or may not work with Google Voice.
No, there is no technical documentation on this. As I mentioned previously, if you need VoIP trunks, then use a vanilla SIP VoIP service provider, not Google Voice.