Yes, there's a mystery. At least two, actually, one in how Han and company got the shields down and another in what happened to Phasma. My point was that Phasma actually having aided the Resistance would account for why weren't shown any of that: If she were aiding the Resistance, then showing all that would be a reveal. So, not showing it would (from a certain point of view, see below) protect the hypothetical twist that Phasma is really with the Resistance. In other words, it would explain why it was handled in a way that makes it seem mysterious. That sort of elision to cover a reveal is an actual trope. Often, though, it telegraphs the reveal. In this case, I'm not quite sure what the truth is, but I like it a lot better now that I see her being in the Resistance as a tenable possibility. The elision no longer seems like so much of a defect to me now, despite my wanting to have seen an actual conversation with Phasma face-to-face.