A Character actor as a Trigger (Can't get Overlap events)

I want a roaming character, that is also an overlap event trigger.
I create a character.
I make sure the character mesh does not have collision enabled (NoCollisions preset; no overlap or hit events)
I make sure the capsule has CharacterMesh preset.
I place the character in a level and begin play, and the character stands there. That’s great!

Now, I change the blueprint for the character (capsule) to select “Custom…” instead of “CharacterMesh” preset.
That’s the only change I make.
When I start playing, the character now falls through the floor (a BSP box.)
Here’s the setting where it works:

Here’s the setting where it doesn’t (as in, falls through the floor):

It seems the Character controller needs the collision preset to be exactly CharacterMesh. Why is that?