I had a task to migrate one of our standard service buses to premium, and when I tried to do so, I faced a very odd error: during the sync step, when the settings\keys\topics are syncing up from the old bus to a new one, they just stuck on 1 entity remaining (total 1) and haven’t moved at all.

After digging in, and setting up logging for the new sbus I noticed the following error:
"ErrorCode\":\"InternalServerError\",\"ErrorMessage\":\"SubCode=50000. Non-partitioned entities are not allowed in partitioned namespace. The property EnablePartitioning must be set to true.
So after re-reading Migrate Azure Service Bus namespaces – standard to premium – Azure Service Bus | Microsoft Learn I noticed this:

And that was it – our new premium sbus was provisioned with 2 partitions and hence had that enabled.
After re-provisioning the new sbus with the partitioning off – everything went by the books.
I just hope MS will be more descriptive about their processes one day and at least show the error.