I have the same exact same issue. Managed Apple IDs are a joke, the only use I have for them is to backup the contacts of my users so that when they get a new device they get their email autocompletion back. Keychain sync is disabled. Using the App Store in any capacity is impossible. There's absolutely no way to use in-app purchases/subscriptions. I got almost laughed at by an Apple rep the other day when I asked if it was somehow possible, as an Apple Business administrator, to purchase extra iCloud storage space for a user.

This is a great article to send to those developers that only use IAP to license the complete version of their apps:

https://blog.kandji.io/in-app-purchases-for-enterprise-buyer...