If you wait for the Update from Windows 10 Version 1809 to 1903, one possible reason is, that you have KFM Known Folder Move installed. This prevents, that you will not receive the install file. And you should NOT install it manually.

Microsoft knows about the bug and there is no workaround. They will have a fix available in late May.

Remarks:

So KFM has issues with Windows 10 1809, so there might still be some issues with KFM and the implementation in Windows.  So as an Windows Insider I  have tested it on one device, but unfortunately, no KFM installed.

Source: Microsoft