We insiders are testing Windows 10 1903, which will be released in April. Incidentally, in the Skip Ahead then the 20H1 (2003). The new Windows 10 1909 insider will only arrive after the start of 1903. Now Mary Jo has been able to find out from her informants why it is.
It is a little complicated. I will try to explain it in general. Microsoft is trying to adapt the basic Windows operating system to Azure. Because it was not identical to the core we use in Windows 10 or in the server version. Here Microsoft wants to give priority to Azure, finally.
The development team always provides Microsoft teams with a Windows Core Platform version in June and December. The individual teams then develop the new features so that they can receive a new version in April and October. By the way, it remains the same: two function updates every year.
The fact that the timing does not work, do not take the version of Windows Core Platform (codename "Vanadium"), which should be provided in June 2019, but jump straight to the Windows Core Platform version in December 2019 (codenamed "Vibranium "). This will be the basis for Windows 10 20H1 (2003), which will be released in April 2020.
Windows 10 1909, for October, will not be based on any new version of the Windows Core Platform. Just take the Windows 10 1903. So, as already suspected, Windows 10 1909 is quite a maintenance update with some new features. But it will only be an isolated case that Microsoft undertakes this chaotic hick-hack. However, it seems you can not guarantee the stability of Windows in the client, server, or Xbox range.
As Mary Jo writes: "The work of the Windows Core operating system (WCOS), originally launched by the Windows Core operating system, is progressing for new devices such as Centaurus, Pegasus, Surface Hub 2, HoloLens 2 and more."