One more comment about that opening a file using the full path to notepad++ works.
If you run
notepad onethen Windows executes the following:
"C:\Program Files (x86)\Notepad++\notepad++.exe" -notepadStyleCmdline -z notepad oneYou can experiment with removing the command line arguments which is how I found the problem is caused by the -notepadStyleCmdline parameter. The -z notepad parameter is also not documented but it appears the word notepad comes from Windows and the preceding -z is used to ignore the word “notepad” that Windows added to the command line.