Thanks Marcia. You have clear the question. This is a bug.

Hi Gustavo.

>> If you have two tables opened (table 'A' and 'B') and have a grid object to each table, if you set a relation between these tables, the child grid table shows only related records. Until now, ok. But, if you release the relation the grid's of child table continues to show only the related records. It must be a VFP bug. <<

Yes it is. It is documented in the Knowledge base. Check out

BUG: GRID: SET RELATION TO Does Not Break Relation Properly
ID: Q148332