I had the same situation after purchasing 24 PBN links, once PBN links pointed to the site all keywords drops suddenly. I wait around 2 months to track whats going on, Stop all link building and sent some social signals to the site around that 2 months,
Then all keywords come into better positions except pbn pointed keywords and pages, Then I contact that PBN provider and try to get all the links that he did, But he didn't give links, anyhow I track those links using Google webmaster,
You know what I experience, Majority of those PBN sites are crap, Seller has no idea what he is doing, sad thing is all links I purchased from BHW and reviewer gave great comment about the Service, Therefore be care full what you are getting, especially from who didn't send any links reports,
I guess in the marketplace need some more strict rules to track services time to time give ta rue comment, Otherwise due to some low-quality service people will lose a lot of money.