When creating a progressive discount promotion the client can set the trigger by collection, by a SKU list or by both.

However, when both are setup on the promotion, the condition that should be either the collection or either the sku list triggers the promotion is actually behaving like only the SKU list is the trigger.

1. Create a progressive discount promotion;
2. On the promo triggers, select a collection and a SKU list with a sku that doesn't belong to the collection;
3. Check on the cart that the promotion is only active when the sku that belongs to the sku list is present.

To enable both triggers at the same time there is no workaround.