This seems an issue in the implementation of Drupal Embeddings Engine or Plugin, unrelated to Milvus. The Drupal code fails to create a collection in Milvus with vector dimension according to the user input. For example, the input is
“OpenAI | text-embedding-3-small" as the "Embeddings Engine", “Dimensions” is set to 1536
While the collection created in Milvus specifies vector field of 768 dimension. It sounds like the dimension is hard coded, or there is a bug in specifying that.
I didn’t find the code of Drupal\vdb_provider_milvus\Plugin\VdbProvider\MilvusProvider->insertIntoCollection() in GitHub. The code of Embeddings Engine or Plugin should reveal the root cause of the problem.