Why does changing a branch plant, on SI orders, result in the default tax rate area blanking out?

Hi

We have 2 companies that transact direct ship (SI) orders to external customers.

The selling BP is defaulted in the order header, along with the default tax rate area from the customer address book. The detail order line defaults to the selling BP, but when this is overwritten with the shipping BP at order line entry, the tax rate area on the detail line becomes blanked out.

This only happens when a local ID is used, but the security has been set to allow access to both companies.

Using a global ID, this does not happen.

Does anyone know what is causing this, as it should remain with the default entry.