If it was completely decoupled back in 2012 then there wouldn't be any advantage to leasing the subdomains or Google being peeved by it enough to take action.
I suspect what is more likely is that the subdomains were separate but not completely unrelated/correlated to the primary domain. Google likely assigned a certain amount of value to the mere fact that it came from the core domain. I think that is what changed here.
Since this also affects leased directories, not just subdomains, there's probably more to it. Afaik, "normal" subdomains haven't been affected the same, only those that were leased out.
I suspect what is more likely is that the subdomains were separate but not completely unrelated/correlated to the primary domain. Google likely assigned a certain amount of value to the mere fact that it came from the core domain. I think that is what changed here.