A single table query can return entities with different partition keys. To get only entities with a single partition key, you first need to determine what the next partition key is with a query for PartitionKey > lastKnownParitionKey that only returns a single result ($top=1). Then you can do a partition scan (PartitionKey = currentPartitionKey) to get all of the entities with this partition key.
If you typically have less than, say, ten entities per partition key, consider optimizing by increasing the first request to get ten entities and discard any that don't have the partition key you are looking for.