minecraft:ancient_city/city_center_3 is one block shorter compared than the other ancient city centers

I can confirm this behavior. You can use the following commands to reproduce this issue.

/setblock ~ ~ ~ minecraft:structure_block{mode:"LOAD",name:"ancient_city/city_center/city_center_1"}

/setblock ~ ~ ~ minecraft:structure_block{mode:"LOAD",name:"ancient_city/city_center/city_center_2"}

/setblock ~ ~ ~ minecraft:structure_block{mode:"LOAD",name:"ancient_city/city_center/city_center_3"}