I have been expecting Microsoft to push out Windows 10 19H1 to release preview ring any day now since the update is all but done, but Microsoft have pushed out a new cumulative update for 1803 instead. The update KB4490481 takes the current version of Windows 10 from 17763.402 to 17763.404.
Microsoft are using Release Preview to test updates to the current version of the OS, rather than testing the next OS update (in this case 19H1) which is a policy I thought might change due to last year’s upgrade issues. This update will probably roll out April 9th, which is the next patch Tuesday.
The next update to Windows 10 is 19H1, it’s due very soon and you can read about the changes in my Windows 10 1903 review article.