I just found something in the udn forum about that:
Collision is disabled on purpose on both channels, because both are being
used in game to represent collision of
world meshes / objects. Yes, it will
allow pawns to move through geometry,
but that’s the whole point, trade
accuracy for speed. If your triggers
should react only to objects on pawn
channel, which is usually true, adding
new collision profile is best
solution. Otherwise, you’ll have to
leave world dynamic channel enabled,
either by modifying engine’s
CharacterMovementComponent or
overriding SetNavWalkingPhysics() in
game specific implementation. We do
later in Fortnite, where collision
settings depends on custom LoD for
pawn, rather than just movement mode.