As a first step, please make sure that Cubasis is signed in with the same Apple Account that was originally used to purchase the IAPs. Then restart the device and try Setup > Restore Purchases again.
I have just one Apple account! But most IAP purchases are made from within Cubasis 2.
So, what I did is install Cubasis 2 alongside again and transfered all IAP’s from v2 to v3. Though every time I start v3, all IAP’s are again missing, and I have to repeat the transfer over and over!
I tried to offload v3 and install it again without succes.
Finally I backed up all my projects and deleted Cubasis 3 completely!
@LSlowak I think I found the solution. It seemed I restricted iCloud access for almost all apps (I believe AI and cloud storage cost too much energy and co-guilty for global warming).
Turning it back on resolved the issue.
It would be smart to have Cubasis check iCloud first before trying to restore IAP!