Last week, Windows 10 Insider Preview build 18362 was released to the Fast ring, and just two days later, it arrived in the Slow ring. The build is meant to be a release candidate for Windows 10 version 1903, but there's only one problem: Slow ring Insiders are finding that it won't install.
As it turns out, there's an issue with upgrading from a cumulative update that's currently in the Slow ring. If you're on build 18356.16, build 18362 will fail to install. Because of that, build 18362 will not be offered to those on build 18356.16 while Microsoft works on a fix.
Microsoft didn't say when the fix will be available, but it did say that it's currently working on it. Being that this is a release candidate, the next step was supposed to be it being promoted to the Release Preview ring.
And really, there's no reason that couldn't happen. Everyone on the Release Preview ring is on 1809, and wouldn't be affected by a bug in build 18356.16. Hopefully, the patch will be available soon and Slow ring users can move on.
Update: An earlier version of this article said that the build was removed from the Slow ring; however, it's only not offered if you're on the latest Slow ring build.