When users to not employ the marker-based multi-tenancy feature, then we add to every query by default the predicate WHERE TENANT_ID_ IS NULL (note that this can be avoided by overriding the engine configuration property tenantCheckEnabled).
It appears that Oracle indexes do not contain values for rows with a NULL index key (or for composite keys, where all columns are NULL), see https://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:2458637000346464034 and https://tkyte.blogspot.de/2006/01/something-about-nothing.html.
We can consider creating the indexes as composite indexes with keys like (TENANT_ID_, 0). That could avoid full table scans for queries where the other predicates do not require a full table scan (e.g. because they are indexed themselves).