Sanctuary increases attracted units limit only if it's larger than your archive. This is annoying because it costs more than archive and is usually smaller than your archive.
It is a design decision rather than a bug. It is intended to work this way, even if it is not optimal. Truth be told though, you can easily create scenarios wherein the sanctuary is used to attract cultists via the map editor.
It does not make sense. Your sanctuary is always smaller than your archive, so its ability to attract units is never used.
Can confirm this is working as intended.
Needs design review in 2018, as I'm not sure of the intended behaviour.
In the event where a player does not have an Archive the Sanctuary needs workers, therefore it is able to attract cultsts independently. To prevent a critical overflow of Cultists either one or the other is used but not both. Essentially it removes the dependency of the Sanctuary on the Archive but doesn't change the Cultist's attraction rate if they own both.