Queries in vfp6 (sp3) don't appear to work as I'm used to (in FPDOS 2.6) if the filter expression involves a related file

the filter 'ANYTABLE.ANYFIELD<>RELATEDTABLE.ANYFIELD' works as expected.

A query based on the same expression selects ALL records (even when the filter shows that this is not appropriate.

I can see that its something to do with related tables as the expression 'ANYTABLE.ANYFIELD<>ANYTABLE.OTHERFIELD' also works as expected.

What is going on and how do I fix it?

Harry