/summon wandering_trader 0 100 0 {NoAI:True}
/summon trader_llama 2 100 0 {NoAI:True}
/data modify entity @e[type=trader_llama,limit=1] Leash.UUID set from entity @e[type=wandering_trader,limit=1] UUID
Poof! The trader llama vanishes!
This does not happen when I set the UUID from @p
I can't confirm this issue. The trader llama doesn't disappear after I leash it with the wandering trader using "/data".