Dummies should indeed be floating (unless you tie them
explicitly, which could be a better idea than letting them
be a random sea of stray capacitance - which short-to-
ground fails to properly emulate).
A pad-stack with Met1 on the bottom might well have
that kind of shunt C, you'd have to check the field ox
thickness and do the calculation. Substrate is assumed
ground, I suppose, but you'd think shunt C would be to
psub! (maybe you use these interchangeably, maybe
the kit does (that would be bad for RF but not out of
the question for a digital-centric kit to insist that psub
is vss!). But you need to be clear in your own mind about
all that, I can only speculate.
A question is, where the fills' bottom connectivity gets
assigned (the to-ground capacitance). If there is a
"helper" symbol / spectre / schematic rep, maybe see if
you believe its contents or want to point to a more
reasonable, modified one (a simple enough matter for
root, if you can follow the chain to the cause).