I have a Managed Metadata column and term set that is configured to allow multiple terms. I built a Content Search Web Part that shows pages tagged with a specific term, let's call it "Foo", but it will only show pages if the term is the first one in the list of terms in the column.
For example, the CSWP will show Page 1 if Page 1 has been tagged "Foo" and "Bar", but it will not show Page 2 if Page 2 has been tagged "Bar" and then "Foo".
I've also tested the query in general search with the same result. And the opposite happens if the term I'm targeting is "Bar" (i.e. Page 2 shows up in my results but not Page 1).
My managed property is "KnowledgeBaseTopic" and my query is: