According to XEP-0369:

Discovering nodes in MIX MUST use a node='mix' attribute in this query. This is to facilitate server implementations that support a single node being both a MIX channel and a MUC room. MUC rooms use this query to return a list of occupants. The node='mix' attribute allows a server to support both MIX and MUC queries without requiring any change to MUC clients. Where a node only supports a MIX channel, the server MUST reject queries without a node='mix' attribute.

Our implementation returns all nodes even when node attribute is empty while instead, it should return an empty list of items (as the MUC component would do when list of occupants is not public).