https://www.theverge.com/2018/4/17/17246170/microsoft-windows-10-april-update-delay

Microsoft was planning to launch its next major Windows 10 update, codenamed Redstone 4, last week. The software giant had targeted April 10th as an internal target to release the update, but a last-minute “blocking bug” delayed the release. In an unusual change, Microsoft has now issued a new build instead of fixing the bug with a cumulative update via Windows Update.

“In certain cases, these reliability issues could have led to a higher percentage of (BSOD) on PCs for example,” says Microsoft’s Dona Sakar. “Instead of creating a Cumulative Update package to service these issues, we decided to create a new build with the fixes included.” Microsoft has not revealed the exact reason for the blocking bug, or why it was discovered at the last moment.