Operating system: macOS Mojave 10.14.5
Graphics card: Intel Iris 1536 MB
Broken: Blender v 2.80 Release Candidate 3
I have to repeatedly enable Mesh Tools add-on. When searching in the workspace, I displayed at the bottom of the filtering section as unknown add-on. I clicked on it and it disappears. So, I open preferences and check the box and it shows up again. I presume it's only a matter of time until it is disabled again and I'll have to repeat the process if I wish to use the tool.
It may disappear during the session or switching between .blend files.