We have a longstanding mismatch between the name of an index as defined in schema and what gets returned from SecondaryIndex#getIndexName(), which for the builtin index impls is the name of the underlying index CFS, of the form <base_table_name>.<index_name>.
This mismatch causes a number of UI inconsistencies:
<idx> must be qualified, i.e. include the redundant table name as without it, the rebuild silently fails
system.IndexInfo (which is also exposed over JMX) uses the form <tbl>.<idx>
here, qualifying <idx> with the base table name is an error.
Generally, anything CQL related uses the index name directly, whereas anthing concerned with building or rebuiling requires the version based on an underlying backing table name.