Invoice post locks material special stock despite late lock

Our business processes a lot of PO's which are linked to a sales order: So we are using the special sales order stock.

The invoice for the PO's are registered and posted on a later date.

While registering the invoices the articles from the PO are locked in table MBEW. This causes all other processes to stop when trying to process these articlenumbers.

We have set the late lock (OMJI) to '2', this works perfectly for regular stock, but it doesn't do anything for the special salesorder stock.

Can somebody help me out how to solve this?