Why isn't the discount appearing on pre-ordered bundles?

From the Shopify Help Center:

Certain features are incompatible with product bundles. Review the following considerations:

  • Bundles aren’t compatible with purchase options, which includes subscriptions, pre-orders, and try-before-you-buy.
  • Bundles aren’t compatible with the Shopify Subscriptions app.
  • Bundles can’t be used with Shopify POS to sell your products in person.

So unfortunately it seems Shopify themselves don’t currently support bundles with pre-order apps. Hopefully this changes in the future.