Here is the code to recreate!!!! It's kind of long and I'm not sure you need everything but it will fail in VFP 6 SP5. I tested in VFP 8 and it does work.

SET SAFETY OFF
lc_sono   = '        67'
lc_item   = 'QAD3672        '
lc_loctid = 'WH1   '
ln_lineno = 1

CLOSE DATA ALL
SELECT 0
CREATE TABLE delme1 (custno C(10), sono C(10), lineno N(4,0))

*-- Add in a bunch of data
FOR ln_cntr = 65 TO 75
   FOR i = 1 TO 15
      INSERT INTO delme1 (custno, sono, lineno) ;
          VALUES (REPLICATE(CHR(ln_cntr), 4), PADL(ln_cntr, 10), i)
   ENDFOR
ENDFOR

*-- Create the index
INDEX ON DELETED() TAG DELFLAG
INDEX ON custno TAG custno
INDEX ON sono   TAG sono
INDEX ON custno + sono TAG custno1
INDEX ON sono + STR(lineno, 4) TAG sono1
USE

USE delme1 ALIAS a_delme1
SET ORDER TO custno1

SELECT 0
CREATE TABLE delme2 (item C(15), loctid C(6))
INSERT INTO delme2 (item, loctid) VALUES (lc_item, lc_loctid)
INDEX ON DELETED() TAG DELFLAG
INDEX ON item TAG item
INDEX ON loctid TAG loctid
INDEX ON item + loctid TAG item1
INDEX ON loctid + item TAG loctid1
USE

USE delme2 ALIAS a_delme2
SET ORDER TO item1

SELECT 0
CREATE TABLE delme3 (item C(15))
INSERT INTO delme3 (item) VALUES (lc_item)
INDEX ON DELETED() TAG DELFLAG
INDEX ON ITEM TAG item
USE

USE delme3 ALIAS a_delme3
SET ORDER TO ITEM

SELECT 0
CREATE TABLE delme4 (loctid C(6))
INSERT INTO delme4 (loctid) VALUES (lc_loctid)
INDEX ON DELETED() TAG DELFLAG
INDEX ON loctid TAG loctid
USE

USE delme4 ALIAS a_delme4
SET FILTER TO NOT DELETED()
SET ORDER TO TAG loctid IN a_delme4

SELECT a_delme2
SET ORDER TO TAG item1 IN a_delme2
*-- Set relations
SET RELATION TO item INTO a_delme3, loctid INTO a_delme4

ln_area = SELECT()
SELECT a_delme2
lc_order = ORDER("a_delme2")
SET ORDER TO TAG item1 IN a_delme2
SEEK lc_item + lc_loctid

? SEEK(lc_sono + STR(ln_lineno, 4), 'a_delme1', 'sono1') && returns .f.
? SEEK(lc_sono + STR(ln_lineno, 4), 'a_delme1', 'sono1') && returns .t.