ME59N Partner missing

Dear all

I have a very strange behaviuor in me59n:

vendor 1234 with partner CP

vendor 5678 blocked

now if i run ME59 with a list of fixed vendor that NOT contains 5678, the purchase order for 1234 HAS partner CP.

If I add in the select also the vendor 5678, partner CP DON'T appear in purchase order 1234.

If now I put away the block on vendor 5678 and run ME59 with both the vendors, the purchase order for 1234 has correctly the partner CP.

I CANNOT UNDERSTAND WHY IS POSSIBLE AND HOW AVOID IT

Can anyone help me??
regards