The issue you are encountering likely stems from resource contention and metadata inconsistencies caused by the large collection (487 million entities with 512 dimensions) in your Milvus instance. When you attempt to load the smaller collection, Milvus may struggle to allocate resources or properly manage channel assignments, especially after scaling query nodes back down to 4. The error message indicates that the QueryCoord cannot find the required replica or channels for the collection, suggesting that metadata synchronization between the QueryCoord and DataCoord may be incomplete or corrupted.

To resolve this, ensure that sufficient resources are allocated for your Milvus instance, as the large collection's size and query node scaling could have strained system capacity. Restarting the QueryCoord and DataCoord services may help refresh metadata and clear inconsistencies. Additionally, consider optimizing the configuration of your query nodes, such as evenly distributing resources or temporarily pausing the large collection to free up resources for loading the smaller collection. If the issue persists, check the Milvus logs again for deeper insights or upgrade to a newer version, as this may address bugs or scalability limitations in 2.3.3.