Why is PostgreSQL 9.5 not using my newest index for ORDER BY, even though it uses similar indices just fine?
In PostgreSQL, an index which is DESC NULLS LAST cannot be used to satisfy an ORDER BY which is DESC NULLS FIRST (which includes ordering by simply DESC because that implies NULLS FIRST).  This is the case even if the column is defined to be NOT NULL.

You could either rebuild the index, or (since you know the column is not null) you can add NULLS LAST to your query's ORDER BY to make it match the existing index.

Note that PostgreSQL does know how to follow an index backwards, so a default index (which is implicitly ASC NULLS LAST) would also be able to satisfy your DESC NULLS FIRST query.  Because of this, it is rarely important to specify DESC in an index, but it can be important to specify which end the NULLS sort to.