When you raise an order with a traceable item set as A Sale May Only Be From A Single Batch you are unable to fully unallocate the stock.  To fully unallocate the stock you have to delete the line from the order.

Support have advised this is as designed and "What if when the time to reallocate comes there isnt enough stock to fulfil from 1 batch" - I would then think that more of a point of consideration and action for the customer and not be a difficult operation to undo (deleting lines rather than amending allocations.)