Your charcoal canister has a vented bottom, so when it gets saturated
with fuel, it will drip out.  This isn't supposed to happen, but I
mention that to explain that the system is not sealed.

Either your canister is oversaturated from age, it has lost it's carbon
granules, there is a problem with overfueling, or a possible
restriction in one or more components.

The vacuum source to the canister is ported off of the throttle body,
so if your injector is leaking with engine off or severely overfueling,
that fuel could find it's way to the canister via the purge hose.  A
more unlikely cause could be a restricton in the fuel tank vent, where
the tank could collapse slightly from purge vacuum, then rebound and
suck excess vapors back into the canister through the throttle body.

Toyota MDT in MO