A Piko climbed up the stump of an already chopped down tree and disappeared once its climbing animation finished.
Steps to Reproduce
Have a chopped down tree, Scare a Piko into climbing it, and It should then climb up the stump and into the nonexistent tree. If it changes anything the Piko also had tea tree seeds in its mouth when it did this.
This issue has not been confirmed by a developer yet.