Ok, found the reason! One CC# sends tuner status off. The Kemper didn't tolerate that for some reason. Since the Kemper now goes into tuner mode on midi volume 0 I don't really need that switch anyway.
That said, I really wish you could delay going into tuner mode by a second or so more! When doing swells it goes in and out of tuner mode like crazy. I'm not sure whether it's due to tuner mode, but the start of the volume curve is very abrupt (I've turned off "Mute Signal" for tuner mode).
Good catch! I am suffering the exact same behavior as described by you and couldn't find out what's wrong. I also do send a tuner status off. So there really seems to be a bug with this specific set up. Thanks!!
Cheers,
Markus