Possibly caused by the fixes of
MC-151047, MC-143699 and MC-152228, the trader llama's DespawnDelay is active, even when spawned with commands or their own spawn eggs instead of the tag being default to 0 similar to the wandering trader, which it would've prevent the trader llama from despawning. Trying to spawn a trader llama with the DespawnDelay set to 0 in the /summon command causes the llama to despawn. The DespawnDelay tag of a wandering trader that has been spawned with commands or with their spawn egg is always default to 0.
- Spawn trader llama (untamed)
→ The trader llama's DepsawnDelay tag is active as it is not default to 0.