Traps are not built as a priority over fortification
Defense construction always has a higher priority over Fortifying UNLESS you dropped a worker on a tile to manually assign him a task, in which case it looks for the nearest task and does that task as it's main focus, even continuing that same task multiple times (fortifying more than 1 wall). Chances are you dropped the worker next to a wall (closer to the wall than it was to the defense blueprint) and he assumed you wanted it fortified rather than the defense built.Whilst a little annoying for you in this instance, this is intended behaviour so isn't a bug.
Another point is that workers always finish a task (if not interrupted) before looking for a new one.When there is a moment where workers do not find any other task than to fortify walls, they will start to fortify a wall and will finish this task (which takes some time), even if other tasks get available after they started.But this doesn't mean they prioritize fortifying walls.
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