Here's what the Release Notes say:
Adds support on Windows 10 Fall Creators Update platforms.
Resolves the issue that certain YouTube videos cannot back up in 2160P (4K) quality.
Minor bug fixes.
My problem wasn't with 2160P (4K) videos or with the Windows 10 FCU (1709), I have version 1706.
I've since moved on to Ashampoo Burning Studio 19. While it doesn't download YouTube videos, it handles all the mkv videos I've thrown at it with the help of Shark007 Standard codecs. (previous forum post)
I've also gone back to Wondeshare Video Converter Ultimate which works every time.