Allocation Unit ID calculation is incorrect?
This seems identical to a case discussed in the comments to Paul's post. He indicated the cause is that the page is no longer allocated (notice metadata IndexId = 1, but m_indexid = 0).
Decoding the allocation unit ID using Paul's script:
DECLARE @alloc BIGINT = 406903719657472;
DECLARE @index BIGINT;
 
SELECT @index =
    CONVERT (BIGINT,
        CONVERT (FLOAT, @alloc)
            * (1 / POWER (2.0, 48)) -- right shift, reciprocal of left shift
    );
SELECT
    CONVERT (BIGINT,
        CONVERT (FLOAT, @alloc - (@index * CONVERT (BIGINT, POWER (2.0, 48))))
            * (1 / POWER (2.0, 16)) -- right shift, reciprocal of left shift
    ) AS [m_objId],
    @index AS [m_indexId];

Result:

This matches the m_objid in your DBCC PAGE output.
Updated question
The calculation is correct if an index ID of 1 is used, again using the formula in Paul's post:
SELECT allocation_id =
    1 * CONVERT (BIGINT, POWER (2.0, 48)) | -- index id
    1913890731 * CONVERT (BIGINT, POWER (2.0, 16)); -- object id


So your actual question seems not to be about the allocation unit calculation at all; rather you are asking why DBCC PAGE shows  m_indexId (AllocUnitId.idInd) = 0 when the page is allocated and associated with index 1 - as shown in the DBCC IND output.
That, I don't know offhand. What I do know is that the calculation works both ways for me, testing on SQL Server 2012 build 6518. Perhaps there is something about your SQL Server version or table that means AllocUnitId.idInd is not being reported correctly. Without specific, reproducible details, you might have to ask Paul himself (though he's currently on holiday).
It's probably some edge case. For example, this post says that there may be differences in m_indexId if the database was upgraded from SQL Server 2000. Try rebuilding the table, that oftens sorts these sorts of things out.