That 97+ generation of carbon canisters had design defects. The air-
gas separator or some one-way vent valve doesn't operate properly and
in some cases allowing the canister to be flooded or the charcoal
disintegrates and sends fine powder all over the evap system. You can
google the TSBs on these. It's seldom the gas cap.

However, you may more likely have sticky vacuum switching valves
(VSV). One vents the vapor into the intake manifold and the other
switches a pressure sensor between tank and canister. It could be
possible that the first one isn't closing properly and the second
isn't switch correctly. Haynes manual details the testing procedures
for these. If they are faulty, try cleaning them by prying open the
metal tabs that hold the port cap carefully. Don't crack the plastic
or break the tabs or you'll have to buy a new valve.

Since the P0446 test involves operating the VSV next to the canister,
that would be a suspect. However, the evap vent VSV (near intake
plenum) has a higher duty cycle and is easier to get to so I'd check
that first and clean if necessary. Autozone can let you borrow MityVac
free (with deposit) so you can tee in and read vacuum. The vent vacuum
should alternate between 0 and about 15inHG and not the 0 to3inHG
kind. You should hear loud sucking sounds as it operates.

Of course, the usual fuel precautions apply.