As shown in the video, my character falls through a moving platform when ascending. I tried thicker convex collision (on all pieces), box collision, scale of 1, CCD on, nothing helped. I also ensured that “Always Check Floor” and “Impart Base Velocity Z” was checked in the Character. Any advice would be appreciated! 
EDIT: VIDEO OF SOLUTION based on insights from IrSoil and Vespineauto007:
It is a work-around… …what if you add a child box collision to the outer ring in your BP. Make sure the collision presets are set appropriately. That why the character cannot fall thru any mesh related collision shortcomings? Maybe???
I think you are saying that you added a simple box collision to the outer ring mesh. If so, that would suggest that maybe the character’s capsule component collision presets are not setup correctly. Maybe???
I can’t see what’s going on exactly, but symptoms looks likes you hit the border of your pit with your head, having no place to move character up and being dropped from your platform.
If it’s really the case, then you can put invisible collision near the wall to workaround this
Thanks so much for your reply and suggestions! The character’s capsule collision when set to the default of “Pawn” still falls through the platform. I tried adding a very thick collision box as a child of the base ring, and that ALMOST worked… except the character now gets stuck inside the collision box instead of falling to the floor… 
That’s an interesting thought, thank you. I will see if I can confirm what’s going on there. I will note though that the pit is using simplified convex collision volumes, so I don’t think there’s any protrusions to catch on.
To @IrSoil 's point… …turn on, or show, collisions in your game. Maybe that will show some kind of interaction with something else that is pushing your character into the floor…
@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
Okay, I think the primary issue is that there are 3 moving platforms, each with collision. The falling through does NOT happen if the character does not stand on the edge of the two platforms. I will try scaling a collision box up during the descent so after all 3 rings combine there is essentially only ONE collision the character is touching…
Still something is weird in that even if there is a gap between your three rings and their collisions, your character is “fat” in that its capsule component collision cylinder should not be able to fit, I assume, thru small collision gaps between objects.
Maybe you can share a screenshot of your character and the three rings such that we-all can see the collisions you have implemented…
SOLVED!!! Praise God! Thank you so much @IrSoil and @vespineauto007 for your help! Both of you contributed to this solution, with IrSoil identifying the possibility of the Character bumping into hidden collision and Vespineauto007 again drawing special attention to collision issues.
As it turns out, the Character was colliding with the landscape’s collision, since the landscape was using a Masked Material to create a hole in the pit region. The edge of the hole was not quite wide enough and the character was physically bumping into this masked collision (as demonstrated in the new video below). After making the hole bigger, the character is no longer forced through the collision of the rings below him!