Vector search recall dropped suddenly today. Should I check the embedding or the filter conditions first?

The data dashboard showed a significant drop in on-site search CTR today. My first reaction was that the embedding model might have drifted. Later, I broke down the vector similarity, metadata filters, and index update times for the same batch of queries and found it wasn't a model issue. Instead, last night's sync script wrote null values in the 'category' field as the string 'unknown', causing…