I had the same issue when I tested a combination of XTC and ADI-8 QS with the HDSPe MADI FX recording interface.
This happens when two different remote control protocols are active at the same time on the same MADI bus using "MIDI over MADI": AUX Device / MIDI remote.
I informed RME about this when I tested a combination of HDSPe, XTC, ADI-8 QS.
If I remember right there is no fix for this available yet.
1. use only AUX device for XTC via midi over MADI. On a preamp you will have to change gain, phantom power and other things more often and its nice to be able to save those settings in TM FX snapshots.
On an ADDA converter there is not so much to be changed and the ADI-8 QS has knobs for changing reflevel in and out on the front.
Therefore there is probably not such a high demand for remote control for a pure ADDA converter.
2. If you should need remote control for one or more ADI-8 QS then use dedicated MIDI cabling then you can use MIDI remote via MIDI. The only limitation might be the max MIDI cable length (6-16m depending on cable quality) compared to MADI which can be 2km between each MADI device.
3. not applicable to you, but to make it complete: If you had a recording interface with 2 or more MADI buses, then you could connect XTC's to one bus and all other devices - which are controlled by MIDI remote - to the other MADI bus and use "MIDI over MADI" for both buses not having to use MIDI cabling.
Nicer would be ofc if both remote control protocols would work independently of each other via MIDI over MADI. Each MADI device has an unique ID on the bus so that it should technically be possible.
X10SRi-F, Win7 Prof, Cubase 9.5, UFX+, Octamic XTC, ADI-2 Pro FS BE/DAC, RayDAT, ARC USB