The vanilla worldgen's template pool 'ancient_city/walls,json' does not include the following template, which is included in the template pool 'ancient_city/walls/no_corners.json':
minecraft:ancient_city/walls/intact_horizontal_wall_bridge
This may be intended; but the name of the pool implies that it's supposed to include all walls, and if that's the case, this one should be included.
Can confirm in 26.2 and 26.3 snapshot 9