From everything I've read here, the thing that's standing out is 301's. They are very often improperly used.
I'm 90% sure this is related to 301's. They are risky at the best of times.
All the websites i used 301s on are stable/increasing, websites that dropped do not have 301s, also websites with no linkbuilding dropped or increasing, this shit is completely random, there is 0 correlation and the comments here show it to. It feels more like an indexing bug if you see how HARD the websites that got hit get hit, dropping out of Google for relevant terms for authority websites, doesn't make any sense. Also it seems that the irrelevant terms either increase or stay stable, which is weird to. Nobody is going to figure out anything, without a Google statement, i suspect a rollback will happen or they take the bug out of this update.
Also fun to know i also run a network of same topic website, with exact same tactics/linkbuilding on different languages, of the about 15 websites, 4 dropped MASSIVELY, 3 increased and the rest is stable. They are linked together to, a.k.a random.