I Read on documentation:

When Visual FoxPro creates an index, it evaluates fields in the first record in the table.
Therefore, if the index expression is a substring of an empty memo field, 
a zero-length index key is created. 

Tip:  
To prevent creating a zero-length index key if a field is empty, 
it might be necessary to enter some temporary data in the field of the first record.

This is true for temporary index that SQL engine build, but it is not true
for the INDEX direct manipulation;
on this case VFP check the first and the last record!
( then it is very hard to control the index key )

clear
ON ERROR ? MESSAGE()
CREATE CURSOR testindex ( f1 char(1),l1 I)
INSERT INTO testindex VALUES ('',1)
INDEX ON left(f1,l1) TAG t1

INSERT INTO testindex VALUES ('',0)
? "reindex"
REINDEX

INSERT INTO testindex VALUES ('',1)
? "reindex"
REINDEX

An side effect is that when VFP build a temporary index,
the length of the key can to be different,
and therefore it can return different results.

Another thing, i look "substring" into the doc paragraph, well:
"you cannot index a SUBSTR(memoField) because SUBSTR return a Memo type"

CREATE CURSOR testindex ( f1 M)
INSERT INTO testindex VALUES ('1')
INDEX ON SUBSTR(f1,5,10) TAG t1

Workaround:
To fill up VFP with CAST(AS C()).

Fabio