Did any of you find a fix for this? I have the same issue. It shows up (and stays) perfectly in the avbutil console, but in the Network Browser it only shows for a couple of seconds after the initial connection and then disappears.
Using a 2017 MBP with sonnet thunderbolt 3 10G interface with a direct connection to a SL3-16R. Running High Sierra 10.13.6.
[Edit June 2020]
Finally fixed it! A firmware update of the Sonnet 10G interface did the trick. Here is a link to the firmware and description, for anyone interested: http://www.sonnettech.com/support/kb/kb.php?cat=537&expand=_a13&action=b1063#b1063
Unfortunately, you can only update the firmware under Windows, so try to find a Windows machine that can actually handle the Thunderbolt 3. Personally I installed Windows under Boot Camp on my MBP to be able to perform the update.