When you do this query:
https://api.appstoreconnect.apple.com/v1/apps/:appId/inAppPurchasesV2?include=iapPriceSchedule
It adds inAppPurchasePriceSchedules objects to the response. But those objects contain no data at all; they just contain relationships links to the manualPrices objects.
As a result, there's no way to include the current price tier in the request for IAPs. You have to make N requests for N manualPrices objects to get N actual price schedules (with actual prices).
The prices endpoint for apps allow you to include=priceTier to see the price tiers. IAPs should do something similar.