Minecraft launcher fails to load on MacOS Catalina 10.15 Beta 4 build 19A512f

Do you mean that it throws an error when you try to start the game from the launcher, or when you first start the launcher? If this is an issue with the launcher itself, I believe it goes over on the launcher's bug tracker

Yes, it's the Launcher.  You can close this as I have created a bug there instead.  Thanks