I reached out to Arturia support. As part of the troubleshooting process I reset my AF Studio, and the ADAT channels were once again available to add to any mix for direct monitoring. I switched my SPDIF output from the default COAX to OPTICAL and then the ADAT channels were no longer available / greyed out. It seems to be a limitation that you can't monitor ADAT channels when using SPDIF OPTICAL outs. COAX works fine, so I bought a digital-to-digital converter ($20) to bridge my COAX out from the AF Studio to the OPTICAL input of my speakers. My setup is unusual as I use desktop speakers instead of proper monitors. Still, this is what fixed the issue.