the bottom plate is too large that it exhibits the antenna effect.
in order to remove this effect, split the cap so that the net area of the bottom plate is also reduced.
in my layout experience, i havent encountered putting antenna diodes to nodes that are not connected to pins yet. usually this antenna diodes are placed near the boundary/cell frame of an IP/megacell.
just split the lower plate and there is no need to put boundary between the 2. put them adjacent to each other and use minimum rule.
it will look somewhat like the attached snapshot.
i hope it helps.