One of the bugs causing issues for Windows Insiders on the fast ring was a shutdown bug introduced in 18995. The bug prevents Windows from shutting down correctly when shutdown or restarted from the Start menu button. The bug was still present in 19001.1 released on Thursday but Microsoft have released a patch 1900.1002 KB4526447 fixing the problem.
My device had the bug but it didn’t prevent the restart of the install.
UPDATE 10/18: We have released 20H1 Build 19002.1002, which includes a fix for the issue causing some devices to get stuck during shutdown or restart.