I have the DAC2 HD (and very pleased with sound) with RPi4 4GB running under Raspian (buster).
I run LMS server and squeezelite on it, and I was noticing squeezelite not starting. Seemed to be intermittent, thought it was just overheating or some other gremlin but having spent a bit of time digging it seems that it always fails on a reboot, but not on a cold start (even after just 10s powered off). Squeezelite doesn't start because it can't find the DAC2 HD, which seems to be failing when starting on reboot.
Not really sure where I should be looking for problem: dmesg or other logs. Happy to post the outputs if you let me know what/where to look.