Inconsistent metadata filtering with serverless index

We currently have a serverless index with the following index stats:

‘dimension’: 768,
single namespace,
‘total_vector_count’: 2096563

When running this query:

                    response = self.index.query(
                        vector=dummy_vector,
                        filter={'nms_id': {'$eq': '990049354070205171'}},
                        top_k=10000,
                        include_metadata=True
                    )

I’m getting 966 items as a result, all match the filter.
If I try a different dummy value, I get 966 results, some of them are different.

Why does this inconsistency happen? This also happens in the pinecone ui.
Thanks in advance!