Correction in transaction O4G1 2-step transfer

In Shipment load confirmation (O4g1) for STO, the system generates 311 and 641 Goods Issue movements. Later, when we try to reverse a loading
confirmation, the system send error message  M7021 deficit of SL unrestricted-use   ... The error is about  quantity in intransit storage location; although the  system creates document with  642 movement, it can't create movement 312 and sends the error Message related above.

If I try again transaction o4g1, the system creates 312 movement in a new document, finishing the reversal. So, at last, the shipment is correctly
recorded in status 2 but I don't understand  why I have to use transaction o4g1 two times.

Thanks for your help.