I've read a few posts about this, including one on this forum (https://www.forum.rme-audio.de/viewtopic.php?id=28387), although there seems to have been some shorting of ring and sleeve going on here, so I wanted to start a new topic for clarity.

I'm running into similar behaviour, using a pair of Sennheiser HD650s with H/P 9+10, and a pair of DT100s on H/P 11+12. If I wear H/P 9+10, but have them muted, I can hear 11+12 bleeding into 9+10. Muting 11+12 in Total Mix stops this. Physically unplugging the DT100s from 11+12 (but not muting 11+12) stops this. The volume control on 9+10 doesn't affect the amount of bleed I hear.

In the other post it's mentioned that this is normal behaviour, because it's a compact unit, but I have to say it's surprisingly loud.. There are no Total Mix weirdnesses going on, and there's nothing on the metering, so it's definitely bleed, but I just wanted to double check this is normal behaviour? Ultimately it's fine, because I'm never using the headphones super critically when I'm recording like this, but I'm keen to know that it's not broken!