My speculation is that they changed something on the backend to disable procs, but they are not even sure about how this exactly works and wich sets are affected by this change.
As for the second issue: lot of people have asked for years some clarifications / insights about how battlegrounds MMR really works..
Sorry mate, you won't get an answer on these two issues. They are going on since years, and yet they are reserving us the usual treatment: deafening silence.
For sure they are aware that there is not an official list of the sets that works in no-proc campaign (wich is both absurd and insulting at the same time for the player base). Lot of people over the time asked to just put an official list somewhere (i mean, not even in the game, a forum post would have been enough), yet as of today there is no such list, and I guess we won't have one ever. You may wonder, why? My speculation is that they changed something on the backend to disable procs, but they are not even sure about how this exactly works and wich sets are affected by this change. Moreover, since they keep adding sets to the game, this mean that at every release they would have to test the new sets in the no-proc campaign, to be sure that they adhere to the supposed standard (whatever that standard is, because as of today nobody really knows it). So they are afraid to write down a list because as soon as we will have one, people will start scrutinizing that list and will open bugs reports against sets that are working / not working in the no-proc environment, generating additional effort on their side on something they clearly don't want to allocate any additional single minute.
As for the second issue: lot of people have asked for years some clarifications / insights about how battlegrounds MMR really works. They decided that is something that we don't have to know. Why? My speculation is that: a) they do not want people to try to exploit the system once they know how the system really works b) the system is far simpler / far more complex than what we suppose, and it would again be a nightmare having to answer to players complaints about why certain people were put together once the rules are clear to everyone.
On that particular point i somewhat can understand their point of view, because matchmaking need to have some rules, but these rules needs to be stretched when there are fewer people queueing for a match, otherwise people would have endless queues waiting for the "perfect match". My guess is that there is some sort of matchmaking only as long there are enough people to be matched following the MMR rules. When a certain waiting threshold is reached on the player side, rules are overridden and you are put in a match randomly more or less, because a poorly balanced match is anyway better than having no match at all.