Thanks for looking into this.
My apologies, I forgot to mention a crucial detail: this issue only occurs in channels with threading enabled. That’s likely why you couldn’t replicate it.
The page_size parameter is ignored when I fetch messages from a specific thread. For example, when I use this endpoint with page_size=5:
/chat/api/channels/64/threads/1526/messages?target_message_id=19205&page_size=5
The API returns the default 50 messages instead of the requested 5, even though plenty of messages are available in the thread. It seems the problem is specific to the threads API endpoint.