Hatchlings get stuck when there's anywhere they can jump. Then, once they're stuck, you can put them anywhere and they won't move. At all.
Saving and reloading the game fixes the problem -- until they get stuck again. They get stuck any time there's a one-high ledge to jump up.
Addendum: with R2-265689 the Bad Hatchling save will crash on load. However, creating a new world and spawning a hatchling with the sandbox reproduces this problem.
Steps to Reproduce
Build a one-high ledge near hatchlings. Wait for them to path to it and get stuck. ALT-Q to move hatchling into the air and watch them defy physics. Or for your game to crash.
This issue has been fixed in either a future version or updated in a previous version.