Short version: tests “yielded no evidence.”
"[I]nterviewer administered polls did not under-estimate Trump’s support more than self-administered IVR and online surveys"
"[T]here is no evidence that higher rates of undecided or refusals to answer (that is, nondisclosure) is associated with level of Trump support"
Trump outperformed live-caller poll estimates by an average 1.4 points, compared to 1.3 points for GOP senate candidates
pewresearch.org/methods/2018/1…
Here's also a more in-depth piece I wrote on the post-mortem:
huffpost.com/entry/polls-wr…
And a piece from 2018 on what pollsters have changed since:
huffpost.com/entry/how-poll…