Could also be that the credit check fails (check if VBUK-CMGST = "B" in SE16).
Also, see this KBA:
1649589 - Saving a Sales Order changes the confirmed available quantity to Zero
Could also be that the credit check fails (check if VBUK-CMGST = "B" in SE16).
Also, see this KBA:
1649589 - Saving a Sales Order changes the confirmed available quantity to Zero