Total Mix (TM version / BF Pro FS driver version / BF firmware version): set to show mono channels (so software return channel 1/2 and physical outputs 1/2 are separate mono channels 1 and 2 .

Reaper: new 44.1kHz project -> creating 1 track and putting a stereo wav (from a CD - so it's CD format) on it. Panning it hard left or right (using the default panner "Project default (Stereo Balance)"): the signal from the other side visually completely disappers in Reaper (track and Master track) but not in Total Mix. It shows a less level but still tells me that there is a signal (i.e. on TM software return channel 2 and physical output channel 2 (which represent the right channel of that stereo setup 1/2) when panned hard left. The issue cannot always be reproduced. Sometimes TM behaves as expected and then again there is this signal indication that mustn't be there. The audio behaves normal - so there is silence from hard panning though TM shows me level on the silent channel.

Gear:
Macbook M1 Pro 2021
MacOS: Ventura 13.4.1

Reaper V6.81