So many little undocumented things about the kemper

I look forward to the wiki.
When your reverb MIX is at 0%, the LED will dim a little, same as delay.
the difference in brightness is not a bug, it represents different states.
for instance, the the rig buttons dim, once you hit the "end" of the list of rigs you can scroll through. the stomp buttons get dim when you switch all stomps off (press STOMPS to do so). same is true for the STACK and EFFECTS section.
hth, g