I just had a thought that might be a bit out there, but given that you're working high production shows it might be relevant. Do you by any chance have MIDI connected to your rig that's part of FOH, keyboards or some other computer / instrument's chain?
I've had previous rigs where a computer would fire MIDI PCs to me to automate changes during songs, etc. Since you have it isolated to the amp and it appears to be somewhat random, this would be a "way in" to your Kemper from other data generating devices, perhaps a CC that Kemper responds to which could trigger the drop in volume.
You probably use the Kemper remote rather than MIDI, but I thought I'd throw it out there just in case.