I don’t know what Apple is doing with audio timing in macOS High Sierra, but they have serious quality control issues in this area. See my post on Avid S3L-X, AVB, and macOS High Sierra for other troubles I’m having.
When Apple released 10.13.2, my Audioquest DragonFly Red started having strange issues. Similar to the clicking issue with AVB, I was having strange timing issues that sounded like phasing, almost like the individual waveform samples were being triggered at a different clock rate than the audio device. It wasn’t constant, but frequent and annoying enough that I gave up using the device. I wasn’t the only one having the issue.
In any case, 10.13.4 fixed my Dragonfly issues, so I’m again happy.
Long story short, if you need use an Avid S3L-X with macOS and playback via AVB, do not install macOS High Sierra. macOS Sierra works fine, and preliminary testing of macOS Mojave gives positive results, but High Sierra has clocking issues that manifest as constant clicking during playback, rendering the audio unusable.
If all you need to do is record via AVB, macOS High Sierra works.
Note, all versions of macOS High Sierra through 10.13.6 are affected.
[Update 2018-09-29] Preliminary testing with macOS Mojave (10.14.0) and 64-channel recording and playback indicates that the AVB problems have been fixed.
[Update 2018-07-28] Increased affected versions to 10.13.6.
[Update 2018-06-05] Increased affected versions to 10.13.5.
[Update 2018-03-31] Increased affected versions to 10.13.4.