I followed the doc and created an app with a new discount type using the sample code (https://shopify.dev/apps/discounts/create#sample-code). I then installed my app on a development store that does not have the Checkout extensibility developer preview (https://shopify.dev/api/release-notes/developer-previews#checkout-extensibility-developer-preview). I managed to create the “Volume” discount as an automatic discount, and when I add an item to the cart in the store, I get a 500 error. If I create a discount with a discount code, I get 500 when I apply it.
I would expect not to be able to create a discount, or not get 500 when using the discount.