Order not scheduling over downtime, it is scheduling before or after downtime.
As mentioned in the following SAP help, the order activities cannot be interrupted by downtimes. This means the activities always come completely before or after a downtime.
If you move an activity in the planning board over a downtime, then the activity "jumps" behind the downtime.
In this help link there is a BADI implementation to change that, however, the custom logic is a responsibility of the customer and we cannot analyze it.
In case of questions regarding the BAdI, please ask our community.