Ok, seems that the trigger components are set up differently for the checkpoint blueprint at least - fix seems to be to set all collision responses of the trigger component to Overlap (some/all were set to Ignore before).
(The collision test logic also seems a little different between e.g. checkpoint BP and the cover BP, FYI)
Edit: regarding the Button Panel, this may or may not be the correct fix, but in the OnComponentBeginOverlap(Trigger Box) test logic, I changed the test for component = character mesh to testing component = capsule component (and likewise for EndOverlap). Again, I have no idea if this is just a 4.18 thing or if my setup is somehow borked.
BP_Camera_change needed (I think) collision set to overlap and also the above change to the OnComponentBeginOverlap test logic (and EndOverlap etc).
BP_Vault_trigger already had the OnComponentBeginOverlap change mentioned above. I changed the collision to overlap but it didn’t seem to work yet.