That title is pretty misleading. The breakage is being reported as being in the default program file associations (Default Programs under Control Panel/Settings):
The file association bug is actually present in 1803 as well, not just 1809. A cumulative update some months ago introduced the bug and subsequent updates never fixed it, and somehow it got integrated into the ISO for 1809 so you'll be fucked on a fresh install. A fresh install of 1803 will be fine... so long as you assign all extensions before updating to the latest build.
It's a complete clusterfuck and makes me wonder why I'm so lazy and incapable of moving to Linux.
On the other hand, this is a comical fail on microsoft's part:
> some Win32 programs can't be set as the default program for a given file type. So if you want certain files to always open in Notepad, for example, you're currently out of luck.
https://support.microsoft.com/en-us/help/4028161/windows-10-...