Could this update be disregarding or down weighting nofollow links? Just an observation.
I know folks think that Googlebot never followed nofollow but in fact it does/did and they do/did affect ranking positively.
I have one (home) page that has around 40% nofollow links to it that has suffered in the last couple of months but inner pages on the same site with <10% nofollow backlinks have improved.
When I look at competitors backlink profiles that have either held position or improved their nofollow links are less than 10%. The other 90% are all (OK mostly) bought or are on sites they control but nonetheless the ratio of follow to nofollow is "better".
I'm mentioning this in case it rings any bells with anyone here.