Shop Pay does not support discounts that are applied through Draft Orders

Shopify Partner
24 0 0
Hey team,
Our Honeycomb Upsell app works through draft orders. 
When accepting upsell offers (based on draft orders checkout) and going through Shopify checkout and Paypal - it works well. 
BUT - it seems that draft order discounts are not applied through Shop Pay.
This is crucial for our client base, could someone help us figure it out?