New
#600
Both bugs exist and are acknowledged by Microsoft as 'Known Issues'.
For the WMP bug Microsoft reports this as a Known Issue in the Nov. 13th CUs for 1809, 1803, 1709 and 1703, saying "After installing this update, users may not be able to use the Seek Bar in Windows Media Player when playing specific files...".
Windows 10 October 2018 Update version 1809 Now Re-Released - Page 52 - Windows 10 ForumsBree said:
The inability to set program defaults was a Known Issue in the Nov. 13th CU for 1809 and 1803. It only affects certain file types, the most commonly reported one being .zip. Many (including myself) could not set 7-Zip (or any other app) as the default for opening zip files. We are still waiting for the fix for 1809, but this has already been fixed for 1803 in yesterday's CU.
Cumulative Update KB4467682 Windows 10 v1803 Build 17134.441 - Nov. 27 - Windows 10 Forums