Cubase 11 drops an audio interface and connects to another

Hi folks -

I use Cubase on a Mac that also runs Pro Tools. I’d like to use the HDIO on my HDX card as my interface (even though the Avid Coreaudio driver isn’t great) but I keep having an issue where it drops the HDX and then connects to another “interface” I’ve used, namely the headphone out of a Caldigit Thunderbolt breakout box. Anyone else experienced this kind of thing? (before I buy a new Avid cable to test that out.)

2013 Mac Pro, 128 GB RAM, Mojave 10.4.6, Sonnet 3 slot Thunderbolt 2 PCIe chassis, Avid HDX and HDIO, PT2020.11, HD Driver 2020.11, Cubase 11.