I was exploring my world and I saw a trader-less trader llama. Out of curiosity, I killed him, and oddly enough, he dropped a lead, despite not being leashed.
Then I killed a second lone trader llama; it also dropped a lead.
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In 1.19.60 a change has been made to cause mobs that were leashed but now have no owner (for whatever reason) to become unleashed and drop leads. This was the fix for MCPE-159617. As a result you should no longer find separated trader llamas with invisible leads. However, the llamas now drop leads when they are loaded after the wandering trader has despawned or been killed, so you may find leads laying around.
When the trader despawns separately from the llama (which might happen if the trader despawns while the llama is in an unloaded chunk), it does not update the llama's "leashed" bit, and any mob with that bit drops a leash on death. This may be working as intended.