Following situation:

1) Product with an active reordering rule

2) on sales order line the route is manually changed to MTO

3) the ordered quantity in sales order triggers the reordering rule (below min. quantity)

4) result: both procurement rules will generate an purchase order

For example: 6 pcs are availlable on stock and we sell 4 pcs by MTO. Reordering rule is set  (min. 5 pcs / max. 10 pcs). The following two purchase orders are generated: MTO orders 3 pcs  and the reordering rule is ordering another 8 pcs. Finally on stock 14 pcs... thats just wrong.

Tested in odoo 12 community.

Stefan