Impacted versions: 9.0

Steps to reproduce:

* Create an MTO product with 
  * Product Type: Stockable Product
  * Routes: "Buy" and "Make to Order"
* Update Qty on Hand to any quantity (i.e, 2).
* Create a sales order with this product and a number <= to the one in the step before.
* Confirm the sales order.

Current behavior:

The created delivery picking appears in state "Reserved" and the current quantity on stock has been reserved, allowing you to transfer that stock.

Expected behavior:

The created delivery picking should be in "Waiting another operation", or at least in "Waiting availability" until the purchase order is confirmed (if you have defined a supplier for the product and the procurement creates the purchase order), which seems to be the current behavior when there's no stock available (is this also correct, as it allows to manually reserve?).
addons/stock/stock.py
res = super(stock_picking, self).default_get(fields)
        if self._context.get('default_picking_type_id') and 'picking_type_id' in fields:
            picking_type = self.env['stock.picking.type'].browse(res['picking_type_id'])
            res['picking_type_code'] = picking_type.code
        return res
    _columns = {
        'name': fields.char('Reference', select=True, states={'done': [('readonly', True)], 'cancel': [('readonly', True)]}, copy=False),
        'origin': fields.char('Source Document', states={'done': [('readonly', True)], 'cancel': [('readonly', True)]}, help="Reference of the document", select=True),
        'backorder_id': fields.many2one('stock.picking', 'Back Order of', states={'done': [('readonly', True)], 'cancel': [('readonly', True)]}, help="If this shipment was split, then this field links to the shipment which contains the already processed part.", select=True, copy=False),
        'note': fields.text('Notes'),
        'move_type': fields.selection([('direct', 'Partial'), ('one', 'All at once')], 'Delivery Method', required=True, states={'done': [('readonly', True)], 'cancel': [('readonly', True)]}, help="It specifies goods to be deliver partially or all at once"),
        'state': fields.function(_state_get, type="selection", copy=False,
            store={
                'stock.picking': (lambda self, cr, uid, ids, ctx: ids, ['move_type', 'launch_pack_operations'], 20),
                'stock.move': (_get_pickings, ['state', 'picking_id', 'partially_available'], 20)},
            selection=[
                ('draft', 'Draft'),
                ('cancel', 'Cancelled'),
                ('waiting', 'Waiting Another Operation'),
                ('confirmed', 'Waiting Availability'),
                ('partially_available', 'Partially Available'),
                ('assigned', 'Available'),
                ('done', 'Done'),
                ], string='Status', readonly=True, select=True, track_visibility='onchange',
            help="""
                * Draft: not confirmed yet and will not be scheduled until confirmed\n