milvus: [Bug]: Range Search Limitied to 6400 Results

Is there an existing issue for this?

• I have searched the existing issues

Environment

- Milvus version: 2.3.2
- Deployment mode(standalone or cluster): cluster
- MQ type(rocksmq, pulsar or kafka): kafka
- SDK version(e.g. pymilvus v2.0.0rc2): pymilvus==2.3.2
- OS(Ubuntu or CentOS): 
- CPU/Memory: 
- GPU: 
- Others:

Current Behavior

When using range search with search param radius , Milvus only returns up to 6400 results even though topk/limit can be up to 16,384

Expected Behavior

I expect to get up to 16384 results, if those results are within the range_filter and radius

Steps To Reproduce

I have a collection with around 1 million records

If I perform a search with no range filter and limit=16384 , I get 16384 results

results = c.search(
    data=[ref_embd],
    #expr="candidate_task_id != ' '",
    anns_field='img',
    param={'params': {}, 'metric_type': 'L2'},
    limit=16384,
    offset=0,
    output_fields=['id']
)[0]

ids = [result.id for result in results]
len(ids)
>> 16384

If I now add radius into params , while keeping limit=16384 , I only get 6400 results.

results = c.search(
    data=[ref_embd],
    #expr="candidate_task_id != ' '",
    anns_field='img',
    param={'params': {'radius': 10}, 'metric_type': 'L2'},
    limit=16384,
    offset=0,
    output_fields=['id']
)[0]

ids = [result.id for result in results]
len(ids)
>> 6400

I have verified that this happens for ALL collections I have.

I have also verified that when I get 16384 records, the largest distance value is < 1.0 , so using radius of 10.0 should still get me back all 16384 results

Another note is search speed is orders of magnitude slower when using range search which I dont understand.

For example when I am using a smaller limit like 30, this same search without radius will run in around 120ms , and when I add radius to params it takes 7 seconds!!

### Milvus Log

_No response_

### Anything else?

_No response_