Well, I've also tested the same with movement type 551.
You don't have any CO document for movement type 551. But if you enter the Order manually during MB1A, then it will appear in accounting entry and BSEG table both.
Have you change anything for your custom movement type after coping from 551 movement type ?
Try once with standard 551 movement type.
Remember the order number will be only updated for transaction event key GBB. It will not update the line for transaction key BSX (as I have shown in my first screen shot).