Option: Open unregistered file types in text viewer -- is not using the selected external program

I tried two different text viewers, but unregistered file types keep opening in the internal Opus viewer.

Option location:
Prefs > File Types > Double-Click on Files > Open unregistered file types in text viewer if they appear to be plain text > Text viewer (blank for default):

Opus v12.29.2

What kind of files are they? The option only applies to files which only contain text data.

.sh, and .properties

I just tried it with two made up extensions (.ddd, and .xyzm) and the external editor opened correctly.
Both .sh, and .properties file types are related to programming -- does it have something to do with that?

The option works here with a test.sh file that contains pure text data.

Are you sure you don't have file types for .sh and .properties? The option only decides what happens for types that don't exist (and files that only contain text data). If file types exist for them, you should edit the file types to change which program opens the files.

I just tried using several file extensions (all files contained pure text only).
.java, and .pl files are also opening in the internal editor.

But if I open the Windows "Properties" for any of these files none of them have an association to a program.

If I rename any of these files to a non-existent extension, they all open in the external editor.

Could it have something to do with WSL (Windows subsystem for Linux), or Microsoft Visual Studio?
I have both installed.

Also: I'm on Windows 10 x64.

Look under Settings > File Types.

If they exist in the list, the option isn't about them, and you can change what they open with there (or via the right-click > Open With menu).

The option exists for randomly-named text files (often config files for different things that use their own special extensions) where there is no file type to edit and thus no way to change what opens them (other than creating a file type for them, which wouldn't be worth it when it's often literally just one file in one place for one program).

The only file type out of the one's I've named that exists in that list is ".sh" (and it has no association within Opus).
Just found out -- .bash files don't work either.
This isn't a huge deal for me -- I'm probably just going to create a few new file types for these files.