No, not this. It's been a long thread so it seems you missed it, but multiple users have now reported this issue on multiple units. Kemper's amp behaves as expected when pushed with an external pedal, but when pushed with an internal drive pedal's volume, it does not behave as expected and its perceived volume decreases.
Who else reported this?
This is the exact issue Lightbox is suffering:
A significant volume decrease at high pedal volume settings beyond +2.5, in combination with an atypical change in sound, that happens with every pedal (not just the KD).