For quite some time, users have been reporting an inconsistent bug in Windows 10 that prevents them from changing the default program associated with a file type.
Windows 10 users are stating that when they try to change the default program used to open a particular file extension, such as .txt files, they are unable to associate it with popular Notepad replacements such as Notepad++.* On the other hand, they have no problem changing the file association to a different one.
"This is a strange bug. It affects some applications, but not others," Chris Hoffman stated in an article on HowToGeek.*"For example, Windows 10 won’t let you make*Adobe Photoshop*or*Notepad++*a default application for images or text files. But you can make other applications, like IrfanView, VLC, or Google Chrome, your defaults."
Hoffman has demonstrated the issues he is seeing in the animated GIF below.
File associations bug demo
According to Hoffman this appears to be affecting users of April 2018 Update, but when we tested this we had no problem making Notepad++ the default program.*This is why this bug is*so frustrating as it is only affecting certain users, certain programs, and under certain conditions that are currently unknown*
BleepingComputer test of changing file associations
In a Reddit thread about their research, other users have express similar frustration with this bug and have seen it also in insider builds.

While Hoffman has stated that this bug appears to have been resolved in build 1809 (Windows 10 October 2018 Update), users are clearly still having issues in that build as well.

Unfortunately, for those who are affected with this bug, there is no known solution.
Bug reported to Microsoft in the past

This bug has been reported to Microsoft numerous times over the past few months through the Feedback Hub in Windows Insider builds.* You can see an example of one of these reports below.
CaFeedback Hub Reportption
BleepingComputer has contacted Microsoft with questions about this bug, but had not heard back at the time of this publication.