Logic Disappearing Sound!?

Sorry, I have no idea.

As noted, I wouldn't upgrade an OS until it's at least half year old. I've been using Mac OS X 10.0 from the very first public beta back in 2000, and this is the most important lesson I've learned.

I don't even have a Mac that can run Big Sur. I have a MacBook Pro 15" mid-2012, the last one ever made with matte display and a built-in DVD drive, and they will have to pry it from my cold, dead hands… ;)

My "mission critical" partition runs El Capitan (including Logic 9.1.8 and Logic X 10.3.3). I also have a Catalina partition where I'm currently exploring the features of Logic 10.6.x, but where I can't use my so called "obsolete" audio hardware due to lack of drivers. Hence El Capitan, until I decide to buy a new audio interface, even though I don't know why I should because the "old" interface does everything I'll ever need.

From what I've heard so far, Bug Su… er, I mean BIG Sur is a nightmare for pro audio:

https://duckduckgo.com/?q=mac+big+sur+audio+dropouts

But frankly, actually it's yearly business as usual though. It was similar reports with Catalina a year ago, with Mojave two years ago, etc.

Rule #1:

If you want to use professional audio software and hardware for a living, never upgrade your system at the "cutting edge". If it ain't broken, don't fix it. Wait until other intrepid pioneers have figured out all the bugs, and the engineers have either fixed them, or other brave users have found workarounds. Then upgrade if you dare.

In other words, if your "old" MacBook has worked for you well, keep on using it for anything "mission critical", and use your new Mini for experimenting until the worst bugs are fixed in, say, about half a year from now.

For what it's worth, here's a word from a random pro expert out there:

Is It Too Early To Upgrade To macOS Big Sur?

Simply put - Yes.

Even though all of Apple’s software will not only be compatible with Big Sur from day 1, including Logic Pro and Final Cut and able to run on Apple Silicon too, even Logic and FCPX users will find that a lot of the plugins that you depend on will not be macOS 11 compatible from day 1 so our advice, even for you, will be NOT to upgrade yet.

(source: https://www.pro-tools-expert.com/production-expert-1/2020/10/21/macos-big-sur-compatibility-the-ultimate-pro-audio-guide-check-it-out-today)

Lucas-JB wrote:

apple wouldn’t launch big sur if they didn’t already test bug etc.

Oh, of course they would. And they do, every year.

Every software has bugs. Apple is setting themselves a deadline for their annual major MacOS release. They will keep this deadline no matter what, including all those relatively "low priority" bugs for this "niche market" that is the audio pros. These folks have learned how to deal with it years ago, and never buy a brand new Mac or upgrade to a brand new MacOS without keeping a fully working and reliable machine as a backup.

If it's a MacOS or Logic bug, it will be eventually fixed by Apple.

If it's a bug with your external audio interface (as you haven't said if you use any), you'll have to wait for the vendor to update their drivers for Big Sur/Silicon chips.