If you purchased a full, paid version of an app (not an app unlocked with an in-app purchase), you can install it for free on other devices with the same account or devices using family sharing.
Unfortunately, the App Store does not support in-app purchases or subscriptions on family sharing (for all apps). See more here: https://support.apple.com/en-us/HT203046
We understand how frustrating this can be for our customers and hope that Apple changes this policy soon. Unfortunately, there is little that we can do until then.