Yeah I spotted this too. Looks like it's treated as a "smaller" duck so isn't pushed away from the opening, but then can't fit through the opening so blocks it for any other small ducks

Hello, this bug has been identified and is currently being worked on for a patch update

The three baby ducks generated from the original yellow duck seemed to have a hard time "recognizing" the kiddy pool path. It seemed like they needed to be pushed into the entrance, rather than getting very close to it and figuring it out, like the other ducklings. They did make it down though! But one got stuck on the rocky path down, never to join his brothers. I haven't seen that with any of the other ducklings, so I'm not sure if these were just funky one-offs or signs of an underlying issue, so I'd thought I'd mention

The key duck is blocking it for me 🔑🦆