hello Paige!
The post with ID 31 is a small post action for pinning the topic globally (ie. Post.find(31).action_code == 'pinned_globally.enabled'), and those posts can’t be interacted with like normal posts, so there’s no way it can have likes.
There is only one regular post in that topic, but it does not have any likes either.
Try Post 33 - that one does have likes and you can see them serialized here:
https://discuss.blues.io/post_action_users.json?id=33&post_action_type_id=2