Good detective work as always.
Unfortunately, I can't test this, I already went back to my working backup but I trust your conclusions.
I just compared with a clean install of 22621, all the basic apps are installed, including SecHealthUI, Snip & sketch etc. The list in start menu is quite full.
Apparently, the installation process for the Apps is changed from 22621 to 25115, as a result many basic apps are missing on the clean install of 25115.
Easy to compare the list of installed apps showing with the P.S. command on the 2 builds, huge difference.
I still think that appinstaller makes the difference.
It shows as Microsoft.DesktopAppInstaller_1.17.10691.0_x64 on the P.S. command on 22621.
and Microsoft.DesktopAppInstaller_1.18.692.0_x64 on the P.S. command on 25115.
But after a clean install of 25115 it still shows the old version. 1.17.10691.0_x64 instead of 1.18, that maybe the whole problem.
Get-AppxPackage -allusers | Select Name, PackageFullName
We'll have to wait for another official MS ISO for Dev Builds.
In the meantime, I hope the guys at github figure out a valid solution. 🤞