USB-C to 3,5mm jack cable is interfering with volume setting

I use an iPad Pro 4th gen as a Spotify output to a soundmixer, for practicing. That used to work fine, but since IOS26, just putting in the usb-c cable (so not even connecting it to the mixer), it interferes with the volume control - either lowering the output volume to 0 or increasing it to the max. You can see it happening even without anything playing: you see the white volume bar at the top of the screen increasing/decreasing. As soon as you remove the cable, the volume stabilizes (the volume bar shows itself briefly and remains constant).

I have already done a full reinstall, but the issue remains. I have read about some issues about a non-Apple cable (ios 18.5), but it remained unclear how the issue was fixed.

Please help!