I just updated to macOS Big Sur 11.3 on a 2019 Mac Pro Rack. All Steinberg applications (Cubase Pro 11.0.20, Nuendo 11.0.20, Wavelab Pro 10.0.70, The Grand 3) no longer pass audio through aggregate audio devices. I have three (3) Lynx interfaces that I’ve used collectively via an aggregate device for years. The 11.3 update breaks this. Other programs, e.g. Spectrasonics Omnisphere 2.5, Keyscape, Native Instruments Kontakt 6 Player, etc., work just fine. It’s just the Steinberg apps that don’t work. If I select an individual interface instead of the aggregate device, everything works as expected.
Aggregate Device is not officially supported by any DAW (even by Logic) since ever. There are some sync issues when using Aggregate Device. Sorry.
It’s worked reliably for me for 15 years, both on Apogee Symphony Gen 1 and now Lynx. No sync issues whatsoever - the devices maintain clock amongst themselves. I have to use multiple interfaces to get the track counts I need. Works fine now with the products I mentioned on macOS 11.3 and did up until macOS 11.3 on Steinberg products.