The vanilla worldgen's template pool 'ancient_city/walls/no_corners,json' includes only the following templates:

minecraft:ancient_city/walls/intact_horizontal_wall_1
minecraft:ancient_city/walls/intact_horizontal_wall_2
minecraft:ancient_city/walls/intact_horizontal_wall_stairs_1
minecraft:ancient_city/walls/intact_horizontal_wall_stairs_2
minecraft:ancient_city/walls/intact_horizontal_wall_stairs_3
minecraft:ancient_city/walls/intact_horizontal_wall_stairs_4
minecraft:ancient_city/walls/intact_horizontal_wall_stairs_5 (Which doesn't exist, but that's its own bug)
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 that do not turn or split, in which case it's missing the following templates:

minecraft:ancient_city/walls/intact_horizontal_wall_passage_1
minecraft:ancient_city/walls/ruined_horizontal_wall_stairs_1
minecraft:ancient_city/walls/ruined_horizontal_wall_stairs_2
minecraft:ancient_city/walls/ruined_horizontal_wall_stairs_3
minecraft:ancient_city/walls/ruined_horizontal_wall_stairs_4