Mannequins can't handle custom server requested swing animation on client.

Mannequin entity, which uses the same model and rendering path as players, can’t display server (with minecraft:animate packet using animation id 0 or 3) initiated hand swing animation. This is not an issue when the same packet is sent to (fake) player entity.

From my quick research into RemotePlayer and ClientMannequin classes, main difference that’s likely to cause it is that mannequin doesn’t call LivingEntity#updateSwingTime() method at all.

Note: This issue requires modded/custom server to be reproduced (vanilla client through!). I’m reporting it as Mannequins were intended to replace fake players, but this issue will prevent that in some usecases.