@IrSoil @vespineauto007 Thanks both of you for your suggestions. I turned on collision visibility and confirmed the falling through still happens even with plenty of distance away from the pit wall’s collision (the visible bump in the video is actually the spring arm’s collision check making the camera bounce back after exiting the pit).
But the fact that the player gets stuck INSIDE the giant box collision after falling through the platform shows that eventually collision kicks in, but only after too much time has elapsed. Perhaps this is an engine limitation, as described in this Epic Staff’s post from 2014: Why do I sometimes fall through a raising platform when I jump on it? - #5 by zeroexception