Recently went through a string of clean windows 10 installs trying to fix this on my end.
Was having trouble setting notepad as the default program for certain types of files, .dll's, .meta, cfg.(worked weeks ago)these types of files found when editing game files for modding some types of games. Noticed after one of my clean OS installs that I could set the default program for those types of files, But then went through the Windows update process to "up to date".
It got to me that maybe I installed a recent update and removing one by one of them, came to KB4458469, upon restart was able to set notepad as default program.
Going into control panel - set your default programs - choose default app by file type didn't help.
Each time I tried to set notepad as default, openwith.exe would come up in task manager/details every time I tried to set it. Also, right clicking the file icon and selecting properties, Opens with was set to "Pick an app" and could not be changed by the button to the right of it.
I do not need troubleshooting help on this but would like to know if there is a way to block this update from automatically being installed.