Indexes must be rebuilt after upgrade from pre-2.3 versions
As part of the implementation of Extended Characters (QuadByteStrings) in version 2.3, internal indexing structures were modified to allow additional bytes in the lookup caches. As a result, if indexed collections are upgraded from a version earlier than 2.3, to 2.3. or later, without index rebuild, indexed queries on these collections will produce incorrect results or generate an error.
There are no errors during the upgrade itself, and auditIndexes does not detect this problem.
When upgrading from a version prior to 2.3 to version 2.3 or later, indexes must be dropped prior to the upgrade and rebuild afterwards.
This step is incorrectly omitted from the upgrade instructions in the Install Guide.