Recent changes to argo fighting, in the past typically you run away and try to catch them traveling between trees but now...
Now you can still do it sometimes, but they are despawning very fast.
Once i had one despawn right after i stomped it in front of a tree, it seemed to change its mind about that tree and started heading to a different one and despawned.
Once it looked like it was heading to a tree and despawned.
Once it despawned right before climbing a tree.
I know they despawn when they enter a area with no trees go to, but all these cases they were heading to a tree specifically. It is more accurate to say all these despawnings was due to a timer, so if it doesn't climb a tree under so much time they despawn regardless if they're heading to one or not. only a guess though.