Ah, so you mean brand terms as in the brands that you carry, not your own? Those are fairly large brands that sell through a number of sites, so you have a large competitive base.
You are seeing the number of different queries rise, but your traffic to the page in question is falling. I would take that to mean that the brand head terms are where you are losing the traffic. You're pulling in longer tail phrases but not enough to recover from the lose of traffic on head terms.
I doubt display has anything to do with this, it sounds like a pure SEO problem. However, with so many brand focuses at such a high level of competition, it is going to be hard to pinpoint what is wrong without an in depth SEO review. I can't provide that to you, but you might look into engaging with someone to do some in depth reviews of the traffic to your brand pages over time to pinpoint the drop started to happen, what changes might have occurred on page to create a change in relevance of the page, if there are new entrants to the market pushing your results down, or if there are any penalties or algorithmic issues with your site that might be impacting the site overall. Those are good places to start.
Best of luck, I know this industry well. Another Kate Morris runs Adore Beauty, so I've kept an eye on this field for a long time.