This is a feature now

src/main/java/com/minelittlepony/client/model/ClientPonyModel.java

@@ -39,6 +39,12 @@ public abstract class ClientPonyModel<T extends LivingEntity> extends PlayerEnti
     @Override
     public void updateLivingState(T entity, IPony pony) {
         isChild = entity.isBaby();
+        /*This was a bug, but I'm calling it a feature
+         * isSneaking = entity.isInSneakingPose()
+         *If a pony is able to get into a 1.5 tall gap whilst sneaking and releases the sneak key,
+         *they will stop crouching. Vanilla behaviour would have them continue to crouch, but that
+         *would look weird, especially with winged pegasi and bat ponies.
+         */
         isSneaking = entity.isSneaking();
         attributes.updateLivingState(entity, pony);
     }