Open ASN during MRP run - deleted order

[osviders]

For long time, I am struggling to understand following MRP behavior, which is causing troubles. 

standard process:

OPEN ORDER -> ASN -> GR -> FULLFILLED ORDER

broken process:

OPEN ORDER -> ASN -> ORDER DELETED BY MRP -> GR DONE FOR NEXT EXISTING ORDER (but total order qty lower that really required)

OPEN ORDER -> ASN -> ORDER DELETED BY MRP -> GR NOT POSSIBLE

How is that possible that MRP touches open ASN? 

Below is example. Scheduled quantity is 560, partially goods received is 280 and open quantity is 280.

MRP changes scheduled quantity to 280, but it should create new line with scheduled quantity 280. 

 Here is the result after MRP. 280EAs is missing.

Thanks for any kind of advice.