Hey folks,
I just wanted to confirm here, that this is not expected behaviour at this time, and that our developers are aware of this and are working on a fix further currently.
Though we are unable to share any estimated timeframes for when this will be resolved, I will be keeping an eye on this internally, and will update in the thread here:
Please also note that this issue is specifically occurring with Plus Plans created in the Dev Dashboard from a Partner Account, so you should be able to test the Billing API with Basic, Growth, or Advanced Plan Dev Stores created on the Dev Dashboard from your Partner Account. If you have any examples of non-Plus Development Stores, that is experiencing the error, please share that in the thread linked above and we can look into that further as well.