This first showed up when I tried clicking on the publisher "Wipf & Stock" link on several of the books in this collection.
https://www.logos.com/product/176495/wipf-and-stock-systematic-studies-collection
The result was an error: Your search "publisher:"wipf & stock"" did not match any results. That's problem # 1.
I tried doing the search manually from the search box: publisher:"wipf & stock"
Got the same error.
I tried searching for publisher:wipf
Got a bunch of results but this was odd; I saw this in the facet panel on the left:

I cannot see any visual difference between those two. I copied and pasted both into a Word document and then searched for the first and it found the second as well. Thus they are actually identical, not just visually so. So why is it listed twice, then, and why the different numbers of books? This is problem #2.
Finally, I realized that publisher:"wipf and stock" might be a better search term, so I tried that, and it worked. But a search for "wipf & stock" should work too, especially since that's how the publisher's name shows up in product description pages for various collections, and in the blog when that publisher is mentioned, and in most names of collections from that publisher (with a couple of noted exceptions -- these inconsistencies should be fixed: Wipf and Stock D.A. Carson Collection Upgrade (3 vols.); Wipf and Stock McMaster New Testament Studies Series (3 vols.); Wipf and Stock John Howard Yoder Collection (4 vols.))
A Google search for "wipf & stock" site:logos.com finds way more hits than "wipf and stock" site:logos.
Part of the confusion might come from the fact that the publisher's official name is Wipf and Stock, but one of their imprints is Wipf & Stock. In any event, a search for either one should find both. Can you make it so?