polanoid

OK in Logic speak, a Multi Instrument is an Environment object managing 16 external MIDI channels, so disregard what I wrote and thanks for the explanation. You're talking about Multi Out Aux channel strips (I guess?) which can of course not be negative-delayed individually as they only transport Audio signals. What if you set a "large enough" negative delay on the Instrument track and insert a Sample Delay plug-in in each multi-out aux, set its "Unit" to "ms" and add a positive delay to adjust the individual aux delay (which would then be the (negative) instrument delay plus the (positive) aux delay)?

Maybe an additional screenshot of your sidechained compressor's settings could be helpful too?

I just tried that, works just fine here. I'm not sure what you mean by that

If you can reproduce any of that, don't hesitate to post here

This was actually quite easy to track down... just input some signal, check which level meters go up, and look which input feeds the corresponding channel strip

So, unless Logic changed the Input of Aux 34 itself due to some issue (did it?), this was not Logic's fault.

Aux 34 (Ld Guitar Delay) routes Input 1-2 directly to Bus 30 which transports it to Aux 32 which in turn routes it to Bus 5, then to Bus 3 via Aux 8, then to Stereo Out via Aux 6.

So which Input are you getting the signal from? Seems your hardware has quite a lot of those

If possible, share the project so others can have a look at it

Is it possible for you to share the project this happens in?

Unfortunately, no

You may want to provide feedback to Apple here: https://www.apple.com/feedback/logic-pro/

Well, if you insist on using a Shift-modifier KC for this, this is probably a good solution 😉

Looks like Logic sends a reset message (Modulation -> 0) at playback stop, so could you try disabling this setting (Settings->MIDI->Reset Messages->External MIDI) ? Also, you could of course try and change your keyboard's settings so that modulation doesn't affect output volume (which is a bit unusual anyway because usually you would use Expression CC#11 or MIDI Volume CC#7 for that)

Well that would use DKD, no? Drummer tracks are assigned to DKD channel strips per default.