index_merge not working in MariaDB
"Index merge intersect" is rarely picked because it involves getting lots of row pointers from two indexes, then intersecting the sets, and only then looking back in the table.

It is always better (as far as I know) to build a composite index:

INDEX(id, flag)  -- in either order


If this is a 1-row table, then the effort of looking in two indexes to ultimately get a single row -- this is a waste.

A side note:  An InnoDB table should always have a PRIMARY KEY.

Put a thousand realistic rows in the table and add a PRIMARY KEY.  Then we can discuss this further.  (But the answer will be the same -- use a composite index instead of hoping for "index merge intersect".)