This appears to ultimately be an error in the level's design - the metablock for the entrance uses the textures that lack the flaps, and this is slightly visible in DOS L3D as well (the upper half of the hinge is missing) - which simply isn't as noticable in DOS L3D as it uses hardcoded indexes from the texture file for flap graphics, unlike Loap which determines them based on the actual textures used by the entrance block. Just needs an adjustment to the level to fix, but also need to look into whether any other levels are affected.