Updating to Windows Insider Preview 18329 seems to have fixed all issues.
Even though the Windows blog suggests that 18329 might have similar issues with Malwarebytes.
Before doing so, I tried doing what David_W mentioned above. I'm not sure if it helped or was necessary, as I did it just before seeing that Windows Insider Preview 18329 was in fact downloading.
Anyway, with Windows Insider Preview 18329, Malwarebytes (3.6.1.2711) is showing as working perfectly. I only report this as an enthusiastic Windows and Malwarebytes user. I realize that vendors are not required to support preview versions of Windows, but it's fun to see these things coming along. Cheers.