After CDRIVER-3148, mongocrypt_ctx_encrypt_init was changed to accept the full command to be encrypted, and generate the listCollections filter based on the parsed collection name. For almost every collection command, the value of the first key is the collection name. However, explain is an exception. It wraps another command in a subdocument like so:

{ explain: { find: "coll", filter: {} }

This wasn't being parsed correctly and resulted in a listCollections filter with an empty collection name.